This is hard because there are children involved and that connects the two of them like it or not. Unless he proves to be a danger and she can get a restraining order and she has sole custody. Yet, he did something bad enough that turned into a lengthy jail sentence which contributes to him being a genuine threat. It's disturbing that when he learned about you he physically abused her. She should have reported that right away. It sounds like she is very afraid of him. It can be dangerous for you if he learns who you are too. And if she did break away from him to be with you, both of you would be living your life looking over your shoulders as long as he is out of jail.