Well OP, one of my best friends started dating a girl I'd liked for about two years after multiple failed attempts on my part to 'woo' her. I still had feelings for her for about the first half year of their relationship, but not any more. If your friend continues to do this he's a pretty shitty friend. If he's willing to try and steal your girlfriend he is indeed going behind your back and also a dick. Even if the only reason for it is that he hasn't accepted that she doesn't like him, he's still a dick and needs to accept that fact and move on. It'll improve his life and your friendship with him.