I agree that Bill's advice, while most honourable, may only encourage her to make a bad choice. Because after all, if it goes wrong, she still has you.

She should know she should not take the risk. Both because of the risk to her as to the risk to the relationship you are having. I would tell her very firmly that if she goes back to him, she should never call you if she ever gets into problems with him again. You don't want that drama.

And if that actually happens and she contacts you anyway, you still have the option to do something, depending on the details. Hopefully she does not takes you lightly and will believe you when you say this. So she knows she cannot count on it.