What can I do to help this boy?

I'm with a great guy. He's a real sweetheart, he has a 6 year old he's been raising for the better part of this year since the mother left him. We've been together almost 3 months. His kid tells me he loves me, my kids love him and the guy himself as well. Despite all of that...baby momma...she's toxic to say the least. Has recently moved back to state. I have told her I would love to be her friend, she has yet to reach out to me. (She left him for this other guys who emotionally abuses her and beats on her dogs, idk why she stays but she does even when she's had every opportunity to get away) as I said she recently moved back and wanted to talk to me. Sure yea I'm down for it. And she proceeds to tell me she's never going to stop trying to get this guy back. In the middle of my living room. I told her she needed to be respectful of his choices (this happend about two months ago, she just moved back 3 weeks ago) she's only seen her son once, and hasn't even been asking about him. Calls my man's today to ask him if he wants to go to a late lunch with her and their son ( lhim and I have discussed this and I'm not comfortable with them alone because I don't trust her and he is fine with that) he tells her no and promptly tells her that their son gets out of school at 2:30 if she wants to go pick him up and see him for the afternoon. She essentially tells him never mind and hangs up. This girl is passing me off, not so much from a girlfriend stand point but a mother stand point. I have to watch this little boy be so sad cause his mom isn't around I'm loosing my patience with her.