Who is this "guy" to you? Is this one of your guys friends and you have been screwing his bytch on the side? Or is this just some guy you know in general?
And in relation to telling him, well, it depends. Many guys are blue pill and believe their woman can do no wrong, of course the chick is not going to admit she's a h0e, so the guy might just try to fist fight you for "spreading lies".
If this guy is one of your guy friends, you should tell him....but that would beg the question, why are you fvcking one of your guy friend's chicks?
Side Note: 97.8% of women in today's market do this. They are with you, but fvcking somebody on the side. No amount of Taiyuu_Otoko gaming tactics is going to change this lol. The market is corrupt. The market is low quality. IF you commit to a chick in a relationship..........please understand.........she's fvcking somebody else or flirting with somebody else on the side at all times.