I agree with Leasha. If you hadn't been in a violent/abusive relationship with him, then it was the wrong thing and it's her life to lead and all that, but because of the nature of your relationship with him, if it were me, I could not live with myself knowing that he was with someone so young who may not have the means to protect herself from him when he starts his crap with her. For now, the relationship would still be in the honeymoon stage and she's going to think he's wonderful, so she may not believe anyone who tells her he's a scum sucking ******* - not even her Mum. But abusers do not change. I would bet my last dollar on him telling her that he was bad in the past, but it was because it was all your fault and that he'd never do it to her because he's changed, he loves her blah blah blah. People like that don't ever admit that they beat their ex because it made him feel like a man. He would do what he could to blame it on you. But what I think is really important is that her family are all well aware of it so when it starts getting bad for her they will be able to see the signs and make sure she's supported.

And think of it this way - by not saying anything and leaving it up to her to find out what a scum bag he is, you would be essentially condoning domestic violence and I know you'd never do that