One of my friends had a 4 year affair with one of my best friend's hubby and it was all found out last year by my best friend. Thank god, I didnt know about the affair because I wouldnt have to be put in this situation of debating on whether to tell her or not. I probably would have told her if I had known because that was her husband and she has two kids with him. Now, that "friend" is pregnant with her hubby's child. I am no longer friends with the cheating friend and the hubby. Turned me off.
It's obvious you care for her since you knew the truth will hurt.. but It'll be better for her in the long run.
I had a friend that wouldnt believe it when people told him his girlfriend was cheating on him.
they got back together.. but i'm not going to say anything and stay out of their business. at least she knows i will always be behind her no matter what
Best answer is one you would answer to your own question..
What would you want your friend to do if it was reversed?
What did you really see? Unless you caught him "in the act" you may not know for sure. If you see him out with another woman, what if it's his cousin?
What if the cousins are lovers?