How to deal with neighbor....?

Alright, my gf lives with me. We both have friends of the opposite . All her guy friends are married and she's usually pretty careful and has never had one cross the line, basically pretending to be her friend, until now. Well, my neighbor who says he's engaged ped by her awhile back while she was out in the breezeway smoking. I think he has seen her out there a few times and decided he's going to start smoking out there too. So he stopped to smoke. I happen to catch part of the conversation one night coming home and she was telling him about how she moved back to be with me, etc. Nothing flirtatious from her, and she was doing all the talking at the time so I didn't get a read on him hiding his intentions. I trust my girl and she has never given me a reason to otherwise. Well, she is a big poker player, and this guy claims he is too. So he told her that he'd invite her to the next tournament that a friend was hosting, and they exchanged numbers. She has kept me a of this since she just met him and doesn't know him well. Well, one night, the guy sends a text to her and it's a pic of his c***. She immediately told me about it. She never responded to his message and I have checked her phone just to confirm i'm not being gullible. She seems completely innocent in this. And this appears to be him trying to be slick or misreading thinking her talking to him and wanting to play poker means something else. I want to punch this a$$hole in the mouth since he has the audacity to pull a bold stunt like this. In fact, since he started talking to my girl when she is out there smoking, he never looks me in the eye when we p each other in the breezeway. My question for the guys is, how would YOU deal with this neighbor? I have not seen him since the incident and have not had a chance to confront him.