What a sticky situation. I had a best friend that was in an abusive relationship. I had to sit and watch until she got her fill of it and called me crying. I finally said "I am on my way" and went and picked up her and her infant son and took them to her mother's house. He had thrown something and hit a ceramic wall decoration that shattered and just about landed on their son. It was a sad situation. The bad thing was I think she knew about her husband abusing his ex before she married him and I guess thought he just wouldn't do that to her.

I would bring up what you know to her but in a casual conversation. Don't be confrontational about it because she will more than likely get defensive. Mention it and then let it go. It will hurt to see her going through it but she has to be the one to make the decision to change her life. It has to be on her own time and terms.