Girls: what do you call the victim of cheating?

Your poor friend! We're not living in the dark ages any more so sex before marriage shouldn't carry the same taboo as it used to. If your friend had have known that he was married she will have fully deserved that insult, but it can't be helped if he deceived her. The wife of this man is foolish - for not chastising him, and for taking him back if this is a regular 'habit' of his.