Been there . . .
A quick story . . .
I am a married man and my friend's wife totally came on to me on night. I said no, because the risk was to great. She decided to make me her conquest and finally after a month of throwing herself at me, my resistance wore thin. We had an intense three month affair that was phenomenal, some of the best sex I have ever had. We kept it secret and ended it as good friends and everything went back to "normal".
Then, a month later she got a case of the guilts and decided to tell her husband (my now former friend). He went ape-shit and my wife found out. I've managed to patch things up with my wife but I lost a good friend.
If you do it, make sure you don't know (or like?) his/her spouse.