“Should I Tell My Friend I Caught His Girlfriend Cheating Or Mind My Business?

I really need your matured advise. I caught my friend's girlfriend cheating (having sex actually) with another guy and she knew I saw her of course.. She's been begging me not to talk.

If I keep quiet now and My guy later finds out I knew about it I will become a bad friend.

If I talk it could be the end of a 4 years old relationship!!!
I'm even scared the girl may set me up if I decide to talk.

Advise please.