Do you think I should warn my past assaulter's new girlfriend of his past?
That sounds dangerous. What makes you so sure that she is not aware of it yet?
Well, to be honest I think she doesn't because he hasn't dated her long enough to tell.
I think that it is kind that you are trying to protect her. You should be careful how you approach it though, she might get defensive if you go about the the wrong way.
I'm going to send her a message. This should be fun.
It might not be fun but you are doing something good. You should try to protect others when you can.