Confusion all around...

This is gonna be long. And I was not there when this happened. I am 20 and My boyfriend is 21. My boyfriend and I have been together for 5 years. He has never done anything to intentionally hurt me. We have had our bumps but nothing too bad. Well he tells me that about a week ago he was at a friends house and everyone was a bit drunk. This girl has already had sex with like 3-4 guys already. Some of the guys were coming on to her and she didn't feel safe. She begged my boyfriend to sit in his lap and cuddle. He made it clear he was with me. And laid out boundaries before hand. He out his arm over her and she say next too him. Before he knew it she kissed his neck twice and she pushed her away and got up and cussed her and defended our relationship. An I alowed to be mad at him? Should I be mad at her? I know he was trying to do a good thing. But I just need advise. He's never done anything like this.