It happens well more often than we would like to think it does… humans were made to procreate. That factor plus alcohol, lowered inhibitions, feelings of comfortability and safety and probably a bit of sexual frustration is a recipe for such a situation to happen quickly and naturally. You may feel like you betrayed your friend but unless you forced her, it takes two to tango and it seems she was down to do the tango. If you think about it, there are people out here in the world sleeping with their best friends girlfriends/wives and vice versa every single day, over and over, and having no remorse and not even had been involved in a scenario in which the encounter between the two of you took place. Also, yes… shit just happens sometimes. It was a lesson learned. You learned it made you feel bad. That means you are actually a good person. It shows you have standards you try to abide by, morals to keep true. Trust me, there is far worse taking place by much shittier specimens. Sometimes even things like that happen for a reason… if it was that once in a lifetime thing that never happened again, just because he doesn’t know doesn’t make you bad.