It would be one thing if the shirt said something like "I am with him" and he has a shirt that says "I am with her" and they wear them at the same time or something that was like coordinating, but that seems very possessive. I mean if he bought it as a joke that shirt and she decided to wear it like once or something, seems okay even to me. But it sounds almost like he is ordering/demanding her to wear this on the cruise? If he is being serious with all this, it is not healthy at all.

He sounds like he is very insecure and his way of dealing with it is controlling male attention/interactions even with a friend who has been a friend forever. There are tons of people of both genders that say someone of the opposite sex is a friend, but there is more to it, but unless there is a reason to mistrust that information, he needs to believe her when she says that guy is actually her friend. Personally, I have had a lot of male friends because I don't like the drama especially when I was in high school/college. DH was wary of me telling him I had a few close guy friends, but he never said anything to me, until he actually met them and was like yeah, okay they are really friends. I was 23 at the time though, didn't live at home, etc. seems a bit strong for the BF to be acting this way as a teenager when his GF lives at home ... 

Jealousy is pretty common and socially acceptable, but it shouldn't be. It is toxic, abusive, and isolating. I have ZERO room for it in my life, but many people tolerate it, for reasons that are beyond my understanding.

Red flag, unacceptable. 

No.. of course this is not normal.

This screams insecurity and is controlling behavior and I'm sure he has thrown in some good gaslighting and making her feel like it is "her fault" for making him angry since she didn't pre-appove the visit to see this guy.. when she went with her brother for goodness sake!

I think you need to have a talk with her about what is "ok" and what is "NOT ok".

Him going through her phone is a sign that HE doesn't trust HER.  If he doesn't trust her.. then they should not be in a relationship.  I would ask if she has ever done anything that would make him doubt her trustworthiness?

Being jealous to a small extent is ok.. but when it comes to this kind of extreme.. it is not healthy... it is a sign of a dysfunctional relationship.

She needs to hear very clearly.. that his behavior is not a sign that "he loves her so much that he can't stand the thought of another guy being near her".. It's a sign that he is so insecure that he has to controll her every move because he is the kind of person who would have no problem cheating given the opportunity.

If your son was doing this to a GF, how would you respond?

I think it's entirely appropriate to talk to your daughter AND his parents about this. Talking to the parents might show you that their values put your daughter at risk just as much as this kid puts her at risk. This isn't okay or healthy, and you need to tell her that. 

ETA: My XH was my high school sweetheart. I had no idea how to have a healthy relationship. When we were about 6 months in, I went to my mom about something I thought was a toxic behavior. She dismissed my concerns so I let it go.

Then something else happened, no one said anything, and I let it go. And I let it go. For 8 years I let it go. Literally the day I told some new friends what was happening and they said "um, that's effed, you need to leave" is when I told my XH I wanted a divorce. After I left, all of a sudden EVERYONE was telling me that they saw his crap behavior but felt it "wasn't their place" to say anything.

You are EXACTLY the person to speak up. Not only are you her mother, but you have lived experience. You KNOW this is unhealthy. It doesn't have to be physically abusive to be bad and wrong. You know that, and you absolutely should follow your gut on this.

I think you should listen to your initial instincts. This is not normal; it's actually really toxic. There are so many variables and ways to handle it, but I'm assuming you don't want to go straight to forcing a breakup? Definitely keep the dialogue with your daughter open; she obviously feels like she can confide in you about him going through her phone etc and you don't want that to stop. But I personally would talk to the boy's parents as well. It's possible that this isn't such an ingrained party of his personality yet, that they may be able to guide his behavior. Or maybe you'll find out that they see nothing wrong with it, in which case he probably is a lost cause. 

This is why I'm such a proponent of teens dating while they're still living with their parents - because you can't protect them from everything, but how else are they going to learn to set boundaries and recognize red flags if not while they're safe at home? My parents forbade dating of any kind and then sent me off to university not knowing a damn thing and I've paid for that naïveté tenfold. 

and I would not put up with this.  THis is very controlling behavior.  My daughter had a bad break up in November.  He would actually say to her, "Equal rights, equal fights."  MEaning he feels its acceptable to hit girls and was removed from ROTC for exactly that.  This kid does drugs and is always suspended or on detention and has bad grades.  I'm so glad they broke up but she is still even now mourning that relationship.  It's so hard parenting teens.  I've definitely been going over with her boundaries and red flags and what constitutes a healthy relationship.    

It doesn't sound normal to me at all.  When I was in high school (which wasn't so long ago as to be irrelevant - I'm still in my 20s) there'd be boyfriends/girlfriends, but we'd also hang out in a larger group and had platonic friends of the opposite sex with whom we did things or hung around.  I don't remember anyone's BF or GF being that possessive, especially at 15.  Her boyfriend is too possessive, and that shirt is over the top. This sounds like a budding toxic relationship.  However, it'd be pointless to forbid her to see him or try to break them up directly - that'll just drive them closer together.  Maybe you could start a conversation with her about privacy and the importance of others respecting one's privacy, being her own person and not letting anyone control her or tell her who she can and cannot see, text or call.  Has her brother noticed any of this?  Sometimes a sibling can provide some insightful information in a situation like this.

Listen to your instincts, they are right on. The boyfriend is way out of line and what he is doing is wrong. How to address it with your daughter will depend on your relationship. I know it has had its ups and downs over the last year. I do think you need to bring it up in whatever way you think will work for your daughter.

Couple things:

1. I totally agree that demanding her phone and going through it are red flags.

2. If we flip the script and it was the girl giving him the t-shirt to wear on a cruise, I think some might think it was cute. AND, if a female had a "bad feeling" about a female "best friend," some would say she's insecure BUT some would say, "trust your gut." Perhaps the boyfriend can see something noone else can. Perhaps the boyfriend knows that this male best friend might have a thing for your daughter but doesn't want to wreck the friendship. There are a slew of tic toks where long time best friends reveal they are actually in love with their best friend. Does the male best friend have a girlfriend or boyfriend? If the male BFF has a partner, then I would say your daughter's boyfriend is insecure and controlling.

I wouldn't force a breakup, but you and your husband could show your daughter how it looks. Your husband could demand your phone and then get mad at you for talking to male friends. And your husband could get a t-shirt made that says "I have a husband. Don't even look at me."

This is a HUGE red flag. My son (now 18) had his first "real" gf at barely 17. They were both equally possessive and jealous. Same kind of stuff you wrote about. Not being allowed to talk to the opposite sex was a HUGE one. Sadly, it eventually got slightly physical; by this I mean a few pushes (and she scratched him once) but nothing else. However, she was about 6 months older and less than one week after my son turned 18; they pushed each other in a parking lot and someone saw. The cops were called and they were both taken to jail for domestic violence. THIS is what it took for my son to realize the relationship was toxic. I had tried telling him my thoughts as I heard their constant arguing (about the jealousy crap), but he didn't want to listen. They are no longer together and all charges were (thankfully) dropped. 

I have no advice other than to be there, but I feel as you do. If I were to have forbid the relationship, they would have just done it in secret. 

ETA: sometimes the only way for teens to learn is to live through it. 

Well I can't speak for teenagers today but what you described there is spot on what my cousin went through at the same age. Except instead of going through phones it was good ol' msn messenger and forcing her to delete all male friends to prove she was committed to their relationship. She ended up marrying the guy and having kids with him. It's only now that they're finally divorcing. 15 years of domestic violence. Not physical abuse like what first comes to mind, it's more a pattern of weaponised incompetence combined with financial abuse, DARVO with a sprinkling of emotional blackmail and love bombing. 

I don't know... to be honest.. the red flags are waving pretty strongly here.

I know it would embarass her daughter to death.. and I'm not talking about an outright "banning".. but I might be tempted to raise my concerns with his parent.. or even directly with the boy himself.

To the boy..

"Hey DB.. I am returning this shirt to you because I find, as DD's parent that this is a totally inappropriate gift for you to give her and I will absolutely not allow her to wear such a demeaning shirt in public.  I understand that you also like to go through her phone and try to forbid her from talking to any other boys and get angry when she does.  Just so you know, I consider that controlling and abusive behavior and my daughter knows my opinion on it.  If you don't trust my daughter to be loyal to you as your girlfriend, maybe you don't need to be dating her? I am not going to forbid that she sees you but I want you to know that I am watching and am encouraging my daughter to have the self respect to not subject herself to humiliating treatment like handing over her phone or wearing a shirt like the one you got for her.. and.. yeah.. here is that shirt.. she won't be able to use it.

If to his parents..

Hi, I'm DD's mom, I am sure you are aware that your son and my DD are seeing each other as GF and BF.  I just wanted to let you know about some things that are a little concerning to me so you can address it if you think it's needed.  Your son has been demanding my dd hand over her phone so he can go through it.. and has been very discouraging of her talking to any other guys.. even ones that are long term friends.. and he apparently purchased a shirt that say "xxx  xxx".  and I you should know that I have forbidden  her from wearing it because I feel the message is demeaning to her and she isn't just a posession to be controlled by someone.  I'm sure that your son doesn't realize how these kinds of things might be a concern, but I want my daughter to feel she is trustworthy and worthy of respect and think those messages undermine that.  I have not forbidden her from seeing him, but felt you should be aware of what has been happening.. as I'm sure you are a parent that cares about your child.. just as I care for mine.

I had my 1st bf at 16 and it lasted 4years. Tbh i do not support and agree with dating as a teen even if the guy is nice and cool (like my ex). I think that we are not emotionally mature enough and will tend to accept a lot of stupid behaviors from bfs/gfs.

I wouldnt allow my children to date until college personally. Your daughter is in a relationship that is quickly going to take a toll on her mental health and you know whats next: bad grades, anxiety, isolation, suicidal thoughts

End this now. She may hate you for it but mother knows best. She will thank you later

You never know about other people.  This description "Good kid, AP classes, Varsity baseball and football player, involved in church, close with his family. We have met his family, all on the same page with values, etc." could be talking about any number of guys I knew, if you change the sports, who physically hurt their girlfriends when we were all in our late teens.  One guy was even so bold as to beat up his girlfriend and her father was the most important politian in our country and she kept it quiet... She got away from him in the end but it took an intervention from her peers and siblings.

Being a key player on a sports team got the guys special priviledges and they though they had the same special rights to women.

Before they married my BIL#1 had to swear to his bovine eventual bride that he would not "so muhc as look at another girl."

He would visit us upon occassion when on furlough from Fort Polk while he was in the US Army.   One visit we went to a C&W music event at an SPJST hall. Several young women approached him and asked him to dance.  He literlly turned away from them, crossed his arms, did not even acknowldge them. I appologized to each one of the young women who approached.

I have zero use for hte cow bride and very little use for my willfully asshole immature BIL1.

I blame my ILs for raising such a dumbass.  Dumbass BIL1's choice of a bride.. is on him.

smh