My initial instincts, like everyone else here, is to just move on and forget about her. But just to look at the other side for a minute, perhaps he should consider telling her what he thinks of her kids' behavior. Tell her that it needs to change or he's gone. Be honest. Let her know that he needs to be allowed to discipline them. I don't know, maybe he has already done this to no avail.
And I admit that I don't know how these things work as I have never been divorced. However, I was one of those kids from a divorce. I can tell you that I was not always pleasant to my mom's boyfriends. Once, we were supposed to go boating with my cousins on a Sunday afternoon. Her boyfriend at the time decided he didn't feel like it. I was so pissed because I was so looking forward to that. I kicked the shit out of his briefcase. It was a nice, expensive one. It was toast.
I also got into a few confrontations (later on) with my step dad. I'm just saying that divorce is not always easy on the kids and just because they are acting out now does not mean they won't end up being productive members of society. Perhaps the good Lord has put your friend in this position to be a positive role model for these kids.