Cheating with a married man

Ok. So just found out one of my friends that I used to work with is cheating on his wife. Not only are they sleeping together, but they are now dating publicly. I'm so mad at my guy friend for doing that to his wife!!! But then again...I'm mad at the girl, (whom I also used to work with) for messing around with a married man. Which makes me wonder...even though it's not my business...in ANY cheating relationship who is more at fault??? It takes two to do that, is the "cheatee" just as much to blame as the "cheater??"