My friend just broke up with her boyfriend...and I really don't like him.?

My friend's boyfriend enrolled in the army and has been sent far away for a little over a year. He called her two years into the relationship (they're high school sweethearts) to tell her that he cheated on her w/ a fellow female soldier. I've never liked him since the beginning because I always felt he was untrustworthy and a shifty character, and turns out that he's cheated in past relationships. He has a questionable past when it comes to relationships with his parents and others, and it really does seem that their relationship just popped up out of the blue. However since he is her first real love, she is totally enamored by him and she doesn't quite see his character like I do. She's implied to me that she may get back together with him, despite past serious and deep conversations between us regarding cheating as one of the ultimate crimes in relationships that is for sure a dealbreaker. How should I tell her how I really feel about this? I've been handling the situation cautiously because I know it's a sore subject, but I really want to let her know that I think he is a sbag and that he doesn't deserve her love or her friendship because she deserves much better than to be hurt so deeply like that. I love her like a sister and am probably taking this more deeply than I should, considering that this does not involve me, but I don't know whether I should be truly honest or just step back and keep my mouth shut. The last time I tried to let her know my true feelings regarding what I think about him, she almost kicked me out of her house accusing me of disrespecting her love for him. Btw he also proposed to her before he left. Help?