Cheating on his hubby

You're at the bar with a couple of friends (you're all married). You've had a few beers, but you're still sober.

Sometime later, your friend Gary calls someone on his phone, telling him/her to come over to the bar.

Fifteen minutes later, a gorgeous young woman walks into the bar, looks over to your gang, and says, "hey, baby," apparently looking to Gary.

"Gary," you ask, "who the heck is that?"

"She's Cristina, my girlfriend!"

Holy Crap. He's cheating on his wife, a good friend of yours.

"Gary? YOU'RE CHEATING ON ANNA??"

"Oh, c'mon, man. Let me have my fun."

You're about to storm out of there and tell his wife, but you remember a favor Gary did for you two years ago.

You had, on one day, left home wearing your usual office attire so your wife would think you were going to work. You actually spent one whole day in your pal's backyard, drinking beer. You started drinking at ten in the morning and stopped at four, so that by the time you hit the road, you'd be sober again. Gary had never told on you.

So, do you tell his wife or not?