Most Helpful Girl
Your not over re-acting. Could the guy have done a crafty one and moved his mouth to hers as she aimed for his cheek? If it was literally a 'peck' on the lips after they had finished singing and they didn't do any flirting or moved away from each other immediately, then it could have just been harmless and friendly.
If the guy is a stranger to her and you at the party, he might not realize she's got a boyfriend there.
Only you know the situation around that kiss, so trust your gut instinct about the intentions and tell her she was out of order to do that if you decide to break up.0THIS IS NOT RELEVANT ANYMORE