If you find out a man is cheating on his wife, it is wrong not to tell the wife.
because
score: 2
By not telling the girl you're taking on part of the responsibility if the man contracts and then passes on an STD to his partner. It would be wrong not to tell her.
Defusing a toxic relationship is easier on both parties the sooner it happens. It's fair to assume that if one spouse is cheating on the other, the relationship is in some way damaged.
You're doing more harm than good if you tell her. Either she never finds out he cheats, and so she's never hurt by it, or she finds out one day and it hurts. Ethically, it's wrong to tell her, because you will be causing her pain that there's a chance she wouldn't have to experience