How do you deal with the ex-wife?

Susan • 25 | 🌈 Lillian Grace | 🍯 Due 9.17
How do you handle the ex-wife?
My boyfriend and I have been together since December and he's been split from his wife since September (still not officially divorced. Ugh.) and she just lives to make his life a living hell. 
She dictates his life by using his kids as ammo. She told him he couldn't see the kids this weekend unless he took them somewhere other than their house (where he isn't allowed to live, but is still responsible for) because she didn't have anywhere else to stay and the decides last minute that he needs to be there first thing in the morning to be there the rest of the weekend.
We celebrated Valentine's Day last night and she knew that so she calls in the middle of dinner and starts a fight with him. I knew getting into a relationship with him that she was crazy, but it's really starting to get old. I just don't know how to handle it.