would you tell someone their husband was cheating??

Ok...so my best friend started seeing this guy and she felt like something was off. So today we decided to do some Facebook snooping and well we found out that he's married. He was leading her on and slept with her and this whole time he's married and his wife is back home on the east coast. She doesn't know what to do. A part of her wants to tell his wife what happened but another part doesn't want to hurt her because she seems so happy and so in love with this asshole. What wou you guys do?? Would you tell his wife what happened or would you just let it go and not tell her??