I hear you bro. If her boyfriend was one of your boys, that would be one thing, but just gym acquaintances, then you don't have any loyalties to him. Unless he would kick your ass! Go for it and then come back here and let us know how it went!
I'M NOT ONE TO ADVOCATE THIS KIND OF BEHAVIOR BUT IF HE ISNT A FRIEND AND THE CHICK IS READY WILLING AND ABLE TO CHEAT ON HIM, HE DONT NEED THE BITCH ANYWAY. IF I READ RIGHT YOU SAID SOMETING ABOUT TELLING HIM. DONT DO THAT. THAT WOULD BE DUMB. FIND OUT IF SHE JUST WANTS TO HIT OR IF SHE WANTS TO LEAVE HIM FOR YOU OR WHATEVER ELSE COULD HAPPEN. JUST MAKE IT UNDERSTOOD BEFORE HAND WHATS GOING ON SO THERE WILL BE NO CONFUSION.