I believe, this is a good idea. You see, men actually are very fragile creatures with lots of strange ideas in their heads. He won't leave You or feel something to her. He just needs to kinda dominate on her, like a revenge or smth like that. I've been in similar situation, but not entirely in sexual aspect. Anyway, he needs to prove to himself that he is "the boss", and I assure YOu, after he does, he'll be a better man)
Good luck to you, guys