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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 should tell the cheater first what your intentions are, and give him the option to tell her himself. If he does not comply, then you tell her.
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