What advice would you give to a friend who was in this cheating situation?

A friend of mine who regularly makes bad decisions with respect to sex recently had an intense makeout session with a girl who is getting married next month. They have been close friends for many years.

The girl who is getting married decided to tell her fiance so that he wouldn’t hear it from someone else, but in her version of the story, the guy drunkenly forced himself on her and she immediately stopped it.

Now, friends of the couple are hearing this modified version of the story, and are furious with the guy. He doesn’t want to tell the truth (which, admittedly, isn’t that much of an improvement) because he doesn’t want to ruin her future marriage (which sounds pretty doomed to me).

I am appalled at the behavior of both parties, but trying not to be too judgmental. How would you respond if you were a friend of this guy?