You have no idea what sort of relationship the OP and girlfriend have....AND you have no idea what sort of relationship the girlfriend and the Ex had. You know nothing about the people involved. You are making assumptions based on your own failed relationships and/or personal issues. And your personal feelings really have nothing to do with legal advice.Possibly I do have an ax to grind, but I was mainly posting to help OP, believe it or not.
OP: Ask yourself this, how would you feel if she were doing this to you? Because, inevitably, she will. Or something like it.
As I said earlier: You have no legal obligations at this point. Run fast, run far.