what is called cheating self?

My friend is really messed up right now. He say's his girl friend finds chatting with other guys on the phone about sex fun, she even sends pictures that are sexy but not completely revealing to them, and he got to find out when he saw some mails she had been exchanging. He say's she hasn't been sleeping around, because the messages aren't about "how good the sex was", but just both parties making up sexy stories up, he had warned her about it some years ago, but it seems she just likes it. He loves her, and i know she loves him like crazy, but he is dieing slowly, and the whole thing is changing this cool dude i know into a player himself, trying to build a defence for his heart. Please what do you guy's think i should advice him?