I am not going to tell my coworker that his fiance was cheating on him.
What reason do you have for not telling your coworker?
I don't care about that kind of thing.
You should reconsider, because while it is well within your rights to not tell him, don't you think that he should know? Ultimately it is up to you, but most people would want to know about their girlfriend's infidelity.
It's my coworker's fault. He should have known she was hot and would turn men on.
If you know that she was cheating then you could at least mention it to him?  What if they get married and she continues to cheat on him, how would you feel?