My friend's fiance made a pass at his ex, but I'm not going to tell her that.
Don't you think that's something it would be important to let your friend know about?
It would be too awkward to tell her.
It may seems awkward at at first, so that's understandable, but you should really tell her. She might be in a relationship with a cheater and you're going to just let her get married to him rather than protect her. She may get upset at you now but she'll get over it and you'll have kept her from being in a really bad situation.
I'm just scared how she'll react if I try to tell her.
Wouldn't it be worse if she found out he was cheating and you knew but didn't tell her? She might be hurt to find out he cheated, but it's better that she know. Besides, you are her friend, so it will be easier to hear it from someone that cares about her.