Why is it so hard to understand why he kissed her back? Can anyone honestly say they've never done something they regretted in the heat of a moment or because it felt right at the time? Even if he wasn't drunk it'd be an understandable mistake. Telling her about it is all you can do. In passion, there is always a battle between logic and emotion. You've logically decided it would be a bad idea to be with her, but sometimes emotion wins without an explination. These things happen. You've done all you can really.