The guy should drop his "girlfriend" and go back to his wife...case closed.
The more he gets involved with this girl, the larger hole he is going to dig himself. What happens if he does challenge his mistress about the guy? Is he going to threaten her, perhaps say he'll tell the husband about the affair? What occurs next is a grudge war b/w the lovers in which your friend's wife becomes informed of her own husband's infidelity, and each couple faces incredible hardship, not to mention the children.
If your buddy screws up, he faces losing his wife, his house, custody of his children, not to mention his dignity and self-respect. I'm not saying he should or should not mention the affair to his spouse - that is his own decision that he will eventually make on his own accord. What he needs to do is get out of this triangle of lovers and mistresses, and concentrate on his own marriage, because he obviously has problems there if he needs another woman. There's no good sense in getting fucked by two scenarios, so my solemn advice is to kick his harem girl to the curb and fix the relationship with his family.
If not for any other reason, tell him to
[i]Do it for the Children[/i]
[img]www.auburn.edu/~littlcb/mountain jew.jpg[/img]