(Drunk)Kissing someone in a relationship?

At this party, I kissed a guy who's dating someone. I know it's wrong and I totally regret it, but I just don't know what to do now. I've talked to the guy, and we made it clear that we're just friends. He said he would tell his gf about it as soon as she got home from her holiday, and by now she should be home. Should I apologize to her? Or should I check with the guy first?I don't know her that well... But I feel like I can't move on; I wake up in the morning and feel soooo bad. Of course she's going to be mad at me, and I accept that, but how should I handle all her friends whispering about it and calling me a slut?