If my significant other would have sex with another man because he "promised" to do her a favor, she would no longer be my significant other. A woman (or man) like that cannot be trusted, and you would not even know if it had happened before, or if it would happen again unless, like this time, they were caught in the act. I would left her there with the sleazeball and told him that he can have her. Once a cheater, always a cheater.