Baby mama drama again

My SO and his ex split up during her pregnancy. They were living together but since she felt like he wasn't doing enough, she left him and moved in with her mother. She was already a bitchy type person and the pregnancy gave her even more of excuse to show her ass. Anyway the last time she pushed him away and dumped him, he was like "fine, I'm done". We met after that and have been together ever since. 
Of course once she found out we were together, she started freaking out. I guess she assumed they would get back together but he now isn't interested in a relationship with her; he says he only wants to coparent. 
We're engaged now and she's still calling him, crying, yelling, and generally makes herself look bad. It's ALWAYS something. Last week she was calling to tell him that she don't like the way his Mom washes baby clothes when she's baby sitting and she wants him to talk to his Mom. Right after she's calling to tell him to talk to his female cousin and tell her to stop talking about her. Like who the heck cares who is talking about her?  Day before yesterday it was calling to ask him what tribe he's from. Then last night it was "my Mom kicked me out of the house with the babies and I have no money, no where to go, and only 20 miles worth of gas in my car". Now she's the type to totally lie. No way her mother would put her and her kids outs. Baby mama (BM) is very disrespectful and loud and verbally abusive so I could maybe see her mother getting fed up with her but not kicking her out. Probably what happened is that BM got mad and left with the babies. She calls SO crying and he's trying to help by saying he can take the kids. She starts telling him all this is his fault, he's a bitch, she hates him, and fuck him she can take care of herself. She hangs up and sends him a text message saying that he ruined the last six years of her life. Honestly she ran him off!
Anyway my problem is with her disrespectful self thinking she can call MY man to solve her freaking problems and her calling at all times of the night. I explained to my SO that she's doing it because she thinks they still have that connection and that he needs to set her straight. They're always going to be connected because they have kids together but if we are getting married, it can't be anymore than coparenting and he needs to set boundaries with her. 
I know this is long but I'm thinking of canceling our wedding if he doesn't get things straight with her. I don't even know if I'm strong enough to cancel my wedding though😢