I fail to see the difference between selling pussy and giving it away. Either way, the vows are broken. Both of them are considered cheating. Frankly, I think the wife who hits on the friend at a gathering is more culpable than a wife who accidentally let the friend slip through her screening net.
Different women have different reasons for doing this. Maybe she's trying to save the house from foreclosure, but hiding it from your buddy so that he can still feel like Johnny Breadwinner. There's no telling, but this stupid "bros before hos" attitude is a good way to get the woman killed, and your friend locked up in jail. Neither of those things are positive outcomes for people you consider to be friends.