Girl kissed my boyfriend

So my boyfriend went out clubbing on Saturday night and this girl (who's his family friend and 2 years younger) got really drunk and kissed him. He said he didn't kiss her back

But then later on in the night, he said 'alright' to her as she was passing by and somehow, she kissed him again.

My boyfriend said he didn't kiss her back but like I'm so confused how it happened again and how he let it happen again.

Is this weird? Should I believe my boyfriend that its not his fault?

HELP