Id let them work it out - but converse with DD about it in depth, so she gets the validation that shes not being treated right. You can bring into the discussion that "someone who is being mean and bullying needs to be called out and told their behavior is unacceptable". We suggest that all the time for skids...same for you goddaughter. But YOUR DD must do this, for her own building up of boundaries. 

I feel that part of the parenting challenge is that you dont want to take away difficulties because they are learning tools. You want to give them the knowledge so they can make the right decisions when you ARENT there to intervene for them.

I hate bullies, they really get me mad, I dont care WHOSE child/daughter it is. I used to watch as SD25 Feral Forger would punch and hit her younger sister and call her ugly and stupid. When I brought it up, FF used it against me and ruined my first valentines day with husband, because when called out, her response was "well Ill just run away or k!ll myself, since Clove called me a bully". Shes also said things like "well no wonder Cloves brother k!lled himself, I would too if she were my sister".

I have a zero tolerance policy for bullies, but seeing as how you want to keep your friendship with the mother, I think this is a good way to go.

Her kids are there, but not BFF? If so, I think you have every right to speak up. This is your home and mean girl behavior should not be tolerated. "If you can't say something nice" would be a good thing for GD to learn. "GD, that's rude/not nice. We don't allow that kind of behavior." Put her in timeout for 5-10 minutes.

Talk to your DD. Let her know that people are not always nice or kind and it's perfectly fine to walk away from that behavior. "I don't know why you're saying that. It's not nice. I'm going to do something else."

With the longevity of your friendship and your gracious qualities, I'm sure you can find a way to gently tell your BFF that her daughter is displaying  mean girl behavior. I'm not a bio mother, but my BFF's kids call me Aunt Aniki. I did call them out on bad behavior and let BFF know. One 'niece' started telling BFF before I could say anything. "Mom, I tried to do XYZ and Aunt Aniki wouldn't let me!" With a bug grin on her face. Testing boundaries? Maybe. She now has a 10yo son who calls me Aunt Aniki and refers to me as her Spirit Mom. *unknw*

First, deal with it in real time correcting GD.  Then call your BFF to discuss the changing dynamic between the girls. Not a blame focused coversation. A solution focused conversation.

There a ton of dynamics in play. GD has a twin. Both GD and DD are girls.  These things happen. In the girl verson of them happening in a situation where two boys with one having twin sister would likely happen.

In our case, my parents had very close friends with two sons about the same age as my brother and me.  We did family trips together for years. One summer they came to my GPs "farm" for a week before the following epic national park road trip.  Where the prior visits had all been a blast, that visit was hell for my brother and me. And even for the younger brother on the other side.  The older brother, my peer, was in a tear up about everything phase.  Not what he wanted to do, tears.  Preparing to go explore on the farm spraying up with OFF to avoid no-see-ums then heading out the door and running into bag worm silk blowing around .... Screams of CHIGGERS!!! and tear. The other three of us just started leaving him to cry and going on about our business.  My dad was fine with it, his dad was fine with it, my mom was fine with it, his mom.... nope.

Make the call, have the talk, and deal withi the GD crap in real time being aware of the risks.

IMHO of course.

Every time I have ignored a bully because I didn't want to make waves, I've regretted it. I'm with the "call her out" crowd, and there's really no reason to report it to your friend. If GD complains to her mother, her mother can contact you. Don't allow her to bully your daughter in your home or anywhere else. 

Completely agree with the above. She's there for weeks! Call her out. You are the mentor and role model here, you should have zero tolerance for the behavior. They are 10? Well, no movie night/Electronics access/pizza dinner then. Or if it really doesn't stop, she may go home early...have a good talk with your friend. She may not be aware.

This week I've had a tough time of it.  I have DD16.  She's going to be come a junior.  She has ex BFF Jessie.  They were friends since 6th grade.  Jessie has developed severe mental health issues and drug/alcohol addictions.  She has been baker acted six times.  She just got kicked out of the highschool.  She was taking pictures of herself performing a sex act and sent it to a bunch of people that she shouldn't have who airdropped it to the whole school.  One of their friends OD'd on drugs given to them by Jessie that turned out to be laced with fentayl.  She's sucicidal every other week.  Their friendship ended a year ago now.  Jessie has a new BFF in my coworker's daughter, Crystal.  I have had Crystal to my vacation home.  I've taken Crytal out on my boat.  But Crystal has become a flying monkey for Jesse.  

THey took took paint to some concrete poles and wrote for free summer ******** , call or text DD16's name and wrote her phone # and they then taunted her with pics of their handiwork.  Well I emailed my coworker and said, "Not cool, man. Reign in your kid."   And I got back, "well it was probably them but we don't know for sure it was them."  

I basically responded back that I'm the advocate for my daughter and I'm going to speak up for her and parent to parent, I felt I had to say something to him.  I told him I hope his daughter escapes the friendship unscathed. 

I know they are a little older than mine.  I have a 9 year old girl and my BFF has a 9 year old girl and they are three weeks apart.  They are now growing apart but even though I didn't get back the response, I felt that I should, I still think it was right to say something.  My coworker was defensive and nasty to me.  I hope your BFF has more tact and grace and realizes that it's our jobs to unleash kids into this world that are going to grow up and not be a&&holes.  We have to mold them and steer them.    

Stop hosting her. Especially not for weeks on end.
 

People will get annoyed with one another if they have different personalities / they're constantly around each other for extended periods of time

They've outgrown the close friendship they once had and now theyre probably better as associates or hanging out with adults moderating  

Love the goddaughter from a distance. 
 

And let your kiddo spend three weeks with kids that are more similar in personality to her. 

A side note my bff from middle school has a daughter near in age as mine 

we get together occasionally and always have the kiddos with us too....been doing "play dates" since toddler ages. 
 

Her daughter is a beautiful sweet young lady but she's WAY more advanced (aka running the streets and you know what that entails) and mine is more studious and a homebody so I know their personalities would clash (or at the least mine would probably come back with navel piercings, crop tops, condoms, liquor, cigarettes, blunts, straight F's) dealing with her for any extended period of time where adults weren't moderating. 

IMHO, the massage to our kids has to be "I do not care what (the other person) did, I care what YOU did. You are now in trouble for what you did. If the other person is a problem, solve it. Or I will.  I suggest that you end any interface with them since you are incapable of making good decisions when  you are with them."

Since her parent isn't there, next time you hear her say something rude, act shocked and say "That was a hurtful thing to say. I know you are a nice person, that's not like you!" See if she apologizes. She may be parroting things she hears online or from mean girls at school, and thinks it's cool or edgy. It will probably take several times of you calling it out for her to get it. She may correct herself. If not, talk to her mom and/or don't host her again. Her mom may be too busy at work and not realize how her daughter is getting, and the behavior may be a bad pattern now. I had to do that with my own kid. I started getting hints that my youngest was becoming a bully and i had a "come to Jesus" meeting with them. I finally told my kid "It's not them, it's you", then discussed ways for my kid to stand up for themselves but be more diplomatic. It's a fine line between being bullied and being a bully sometimes and kids need guidance, whether they are too assertive or not assertive enough. 

Protect your bio child. For sure,  speak up and correct the behavior. ADD "WE do not treat each other that way in OUR home"