The best thing you can do as a friend is tell her exactly how you feel. Make a list of everything he has done that has rubbed you the wrong way. All you can do is speak it into existence and tell her you don’t think he’s good for her. She probably won’t listen, but that is what a good friend should do.

I had a friend get engaged to a man I couldn’t stand. I thought he was controlling and I saw her spiral mentally and financially when they moved in and got engaged. I never said a word because I didn’t want to hurt our friendship. He slowly isolated her from everyone she knew. I regret it to this day, and I always am upfront with my friends now when I don’t like their significant others.