I'm wondering why you'd want to be involved with a guy like that. If he does it to her, then he'd probably fool around behind your back too. Sometimes relationships end and you grow more attached to someone else, but if this is the case he should be willing to end it with the other girl. Personally, I'd want whomever I was with to show a bit more integrity than he is. If he's not willing to end things with his current girlfriend, then I wouldn't accept his shows of affection. Just because he's making the first move doesn't mean that your allowing it isn't an action of itself. By doing nothing to 'stop him,' you're making an active choice to allow things to continue as they are. I wouldn't want to be involved in anything like that, by action or inaction.