As a parent, it's distressing and frustrating because really what you've done is all you can do. You can advise and encourage him to leave or do something about it, but in the end it's his decision. It's hard because you just cannot control the situation and make something happen even if you are indeed right and know what would be best. I suggest shielding yourself from this and learning to cope with the idea that your son is with someone you believe is a cheater.