It is no business of your,because they seem to know eachother before they got married.I say let your friend find out himself

ifystallon:
Should I tell my best friend, his wife is cheating on Him?

Please do you think I should. I am having mixed feeling because i don't want to end up being the enemy.

Lets forget about spirituality here u can handle it personally, but first confirm if your suspicion is true, confront her n ask her to change but be careful shld in case she is like pharous wife, she ll try to turn her hubby against u. But in all n all if she doesnt stop u hv to tell your friend u might be saving his soul.