I think you should talk to your girlfriend about it because there may be more to the story. Just to clear the air, it's better if you talk to her about it. As for your friend, again there is always more to the story so if you want to talk to him, that is your choice. If he was the one who instigated the kiss, then I don't think I would consider a person who does that behind my back a friend. I would personally distance myself from that person. A friend respects you and does not take advantage of a situation like that. I would suggest you cut ties with that person and leave it at that if that's the case. If it turns out to be the other way around, it will be your choice to continue the relationship or end it.