Your daughter sounds like she is on a great path to success in life. I would honestly step back and let her make that decision. I would hope that her dad would see that even if a relationship between sd and sm doesn't work out, the relationship with him could still exist.

If your Ex is asking you to intervene remind him that your daughter is an adult and perfectly capable of choosing with whom she has a relationship. 

Your DD20 is an adult. Stay out of what isn't your business.

If her Dad insist on having to only see his daughter when and only when his wife is present, so be it. I'm sure SM is the one making the demand. Sm is going to lose this one and Dad will lose what little relationship he has with his daughter. His choice. 

No mother should try and force their daughter into subjecting their selves into such a vile ordeal. Screaming table turning loud mouth drunks? Seriously. I wouldn't put up with this wacko woman , why would I encourage , let alone, try an force a young adult to? Luckily the young lay is now an adult and free to make-up her own mind without being under pressure of a CO. 

Yes, she does risk the lost of having her father in her life, and for that I am sorry. But what  kind of relationship could they possibly have when the nasty woman must also be present? When DD marries and begins to have your grandchildren, are you going to try and force your DD into subjecting the kiddies to this extreme behavior of their  step grandmother just so they have a paternal grandfather? I'd hate to think small children would be on the opposite end of a flying table.

Stay out of it. Yes, you Dd is hurting over this I am sure, but whatever decisions and actions your DD makes from here on out are hers to make. My suggestion is you cease discussing your ex with your DD when it comes to this. If Dd needs someone to help her sort of her feelings on her father, recommend therapy. It sounds as if the poor young lay has certainly been put through the runner by her father and SM. Perhaps counseling could help her build coping skill toolbox to deal with her father without involving you. 

Give her a hug and quiet support in whatever she decides. 

I agree, if DD wants to see her dad, she can decide.  

Since you and your ex get alone farily well. I do too with mine. Anyway, maybe talk to him---HEY x, are you reallllly ok with the marriage your in? I am concerned. Our daughter loves you very much. (dont use but)

Our daughter also knows when to back away from unhealthy situations. Watching sm's name FLIP the table is grounds to stay away xh. 

Until sm has progress with what ever it is she is dealing with you relationship with everyone, daughter and me will change in the following way. WE will not put ourselves near it.

Screw dad and daughter going to therapy...his wife needs it. Until she is in treatment of what ever kind,,,stay away. Keep door open for dad when he realizes what his wife is.

Be proud of your daughter for knowing to walk away from that mess. 

Tell exH that you want nothing to do with any of this, DD is a grown up who can and will make decisions for herself. It is up to him to have a relationship with his daughter.

Tell DD the same thing. It is up to her to have a relationship with her father. Don't get caught up in the middle of this. Let them decide what their relationship looks like.

his child and that will be his choice. My ex has done something similar. Our kids (23 and almost 20) basically have no relationship with him because he insists that his wife come along. He has met our older grandchild twice, has never met the baby (DS has two kids with his wife), refused to come to son's wedding despite his wife being invited because she would uncomfortable and when exes stepmom passed away a few months ago he barely said a word to either of our devastated kids but got angry when someone asked our grandchild if he was going to say hi to grandpa and he started looking for my husband, who is the only grandpa he knows. He sent an email making it clear to me that my husband of 17 years should never be called grandpa. I deleted it. I am there for my kids with advice and a listening ear but I will not get dragged back into co-parenting. 

Your daughter is grown she is not obligated to see or be around SM. If SM is as toxic and full of conflict like you stated than I wouldn't want my child around her either. Either way your daughter made the choice to not be around her and that is her right. She wants a relationship with her father not SM and not sure why he would want his wife around knowing all the awful things she has done. I'm guessing that his wife has him by the balls and he is bending over backwards to please her so she doesn't wipe him out financially.

So saying all this to let you know that your daughter has a good head on her shoulders and she doesn't owe SM a damn thing.

stepmums like that give us a bad name! Your ex wants everyone to be happy and get along. But not everyone in life can get along. Your ex will loose his daughter in the process. He either needs to sort his partner out and seriously try and fix the situation to atleast a point they can be civil or he is going to loose either his child or his partner. But at this point he cannot have both in a situation like this