Young couple [M20] [F20] attended at a party and a cocky guy [M30] hit on her in front of helpless boyfriend. She ends going away with the cocky guy.

I witnessed this situationship one a a party and was really intrigued by this scenario. Young couple in their first 20, she was very pretty and he was shy (she was out of his league). At the party a cocky-arrogant guy hits on her. The boyfriend, intimidated, did not know how to react. At first she appeared uncomfortable because her boyfriend was there, but seeing that he was not able to react and did not know how to assert himself, she started flirting back. The cocky-guy behaved as if the boyfriend did not exist. When bf tried to join in the conversation, the cocky guy looked at him expressionlessly, then went back to talking to her, which was increasingly relaxed and involved. He started to put his hands on her hips, to caress her back, then took her dancing. After half an hour she returned to her boyfriend and without looking him in the eye (I vividly remember this one particular) said : "I'm a little tired I'm going home, he'll take me. You can stay". She kissed him on the cheek and went out with the cocky guy, who was shit-grinning a him. Did someone happen such scenario (true stories) or something similar WITHOUT or BEFORE being in a cuckold relationship?