how should I act?

So one of my best friends recently started going out with one of my exes....she knows quite a lot of what we've done, and even that he's forced himself on me a couple times. My relationship with my ex is over with, since I do have a boyfriend that makes me really happy. I know I should be happy that she's happy, but I just can't help but think that maybe he'll hurt her like he did me.ges very controlling and gets mad very easily and makes stupid arguments. I know..."people change". But...I just don't know about him. He goes around a lot and I just don't know what to say to her. I want her to come to me as a best friend and act like normal, even ask for relationship advice (because I'm in one too, not just because I dated him) but I don't think it will ever get to that. What should I do? And how should I act around them?