Hey girlie! This is a pretty tough situation. If his girlfriend is a really good friend of yours then she has a right to know. If not, don’t get involved. Hopefully he’s a good guy and will tell his girlfriend himself, without mentioning your name. Avoid at all costs being alone with him again. Even if they do break up, you should steer clear of guys who kiss other girls while they still have a girlfriend. You say you might like him, well consider the circumstances. You were at a party, dancing, having a good time. He showed you that he was interested, what girl doesn’t like attention? Just remember that he has a GF. This may not be the first time he has done this, and may not be the last. You wouldn’t want a BF to do that to you. I recommend staying away from taken guys who flirt with other girls. Good luck babe!