Overheard my (M23) girlfriend (F24) advise this to her friend: "don't tell your boyfriend that you kissed that other guy yesterday".

We've been together for 3 years and currently live together. I wasn't even trying to overhear, she was on a call with her best friend and had her on speaker.

That friend has recently entered a new relationship (maybe 2 months at most) and yesterday she was out with some friends at a club. One of the guys there didn't know she had a boyfriend and was trying to make moves, touching, dancing, etc At the end of the night, he asked for a kiss and she didn't want to kiss him on the lips so she kissed him on the cheek.

My girlfriend, upon hearing this wonderful story about her friend flirting with another guy and giving him a kiss while in an official relationship, just told her "oops, better not tell this story to your boyfriend" and they just laughed it out and moved on.

And I am just sitting at my desk hearing all this, completely dumbfounded. Maybe I am lacking details, but I can't possibly ignore the advice my girlfriend gave her friend, especially considering that there have been some occasions where I had doubts about my GF's interactions with other guys on nights out but eventually chose to trust her after numerous discussions.

Is this none of my business and I shouldn't have been listening in on her at all? What to do?