If you were close friends with two people that were married and you accidentally found out one was having an affair would you a) tell the other person about it b) confront the cheater and tell them you know about what they are doing and think it is wrong or c) try to stay out of it and hope the cheating partner eventually comes clean themselves?

I asked this question on another site and most people said C but some said A and would want their friend to tell them about it if it were the opposite way around and they found out their partner was cheating and I am curious what option most people would go with here.