Would you tell him?

My best friend cheated on her boyfriend about a month or so ago and the guy she cheated with told me. Her boyfriend isn't exactly a close friend but he's a acquaintance to me and I feel bad not telling him this. I know it's not my relationship but if it was me I think I'd want to know. I tried telling him just to break up with her because they don't see each other often and just other excuses. I don't want to be the one to tell him his girlfriend isn't being faithful but I think he deserves to know. She doesn't know I know either. I'm stuck in the middle, what do you think I should do?