Uh...sorry but...to the answerer before me, you don't see why she should tell her "best friend" that the man she's about to marry is unfaithful?
Lets just look at this situation logically for one second. If you decide to not tell her, she will be marrying a man who is clearly a cheater, a dirt bag, a man who cheats on her, a man who will cheat on her again. He is not faithful.
What if you knew her car brakes were broken and you allowed her to drive her car on the freeway anyway? If so, then maybe the term "bestest friend" should not be used to describe your relationship anymore, because friends look out for each other and alert each other of dangers. That is after all the definition of a friend.

Now, if my best friend told me about this, I would be more relieved that they had told me so that I would avoid marrying a scum bag. I would be less mad at my best friend and more mad at my soon to be husband. Again, however, I would be more RELIEVED that I averted marriage with such a man who would cheat on me with my best friend.

If you keep quiet about this, she gets married to him, and they pop out a few babies, and he cheats on her again, and she finds out, guess what? Now they're divorced with young children.
If she marries him and he cheats again, which he will, trust me, he will. They will divorce in a matter of a few years you can bet money on that.
I would tell her. I would alert her that the man she's so in love with and banking on spending the rest of her life with, is a scum bag who doesn't have her best interest at heart.
I hate when people say "Don't let them know they cheated, it won't help anyone, it'll just ruffle up the feathers." Excuse me? Uh, cheating ruffled up the feathers. I would rather KNOW.