<human>: Ok, I'll keep this short and sweet. I'm a college student and a girl I knew from high school (but had no real interaction with before) visited her friends who also attend my school this past weekend. We had a fun night, I slept with her, and she left the next day with the implication that it might happen again. The next day I went on her facebook and found out she has a boyfriend even though I specifically asked her if she was taken. Obviously, I will no longer be pursuing her, but now her friends (which are mine as well) aren't speaking to me and putting the blame on me. I feel terrible for what I've done, seeing as I've been on both sides of betrayal. I never wanted to be apart of cheating again. Do I have an obligation to contact her boyfriend and let him know what happened, even if it blows back on me, or do I make her do it? Shit do I just let it go and say nothing? I have no idea what to do, please help.

Can you succinctly summarize the text in 1-3 sentences?
<bot>: I unknowingly slept with a girl I know who lives out of town and she has a boyfriend. Do I contact him or do I let it be and lay low?