How do you tell someone their partner is cheating?

Oh my. This is a problem. Different scenarios abound:

1) Don't tell him. When he finds out you knew but didn't tell him, he'll call you an asshole.
2) Tell him. He'll call you a liar AND an asshole. (Or, he'll believe you, and still call you an asshole for making him unhappy).
3) Talk to her. She'll tell him you accused her of cheating because you don't like her, and he'll call you an asshole.
4) Don't talk to her. He'll find out, and we'll have the same situation in scenario 1.

See the problem? No matter what you do--you'll wind up with the same thing. You need to stay out of it. Until she hits on you--it doesn't concern you.

Jam it back in, in the dark.