The situation does seem a bit complicated to be given simple statements like "if she'd go behind his back she'd go behind yours." But at the end of the day, even if the situation is complicated, once a cheater always a cheater is the credited response. Women on the market are ALWAYS ALWAYS ALWAYS looking to trade up. And if she doesn't consider herself off the market when she's married (even if it's a ****ty marriage) she never will. HTH
P.S. Azalin makes a good point. Why is it the guy who wants the divorce? Women initiate 70%+ of divorces. When the man initiates something is up.