I'm friends with this dude since high school, we always hung out together and were very close. He confided very personal stuff to me, saying I'm his only friend. 

When I got into college, I instantly clicked with this girl and we became very good friends. We traveled together, went to parties, etc.

Eventually, being both my friends, they met and soon began dating. So far so good, the only problem is that the dude is a total shitbag. He made out with her childhood friend in front of her AND SHE FORGAVE HIM.

OK cool, however, after a few months of The Incident, girl traveled to the countryside while me and boy went to a party. Guess what? He cheated on her again, with his ex.

I didn't tell her and told him not to either because it would ruin our friendship. And to me it would be a unnecessary stress since she has forgiven him before. 