Caught my partner with someone else

Not intending to go into specifics, but my partner and I have been dating for a few months. We’ve been on good terms and recently he attended some event with friends while I stayed at home to catch up on deadlines and school work.

A mutual friend caught him making out with another person and posted it on their closed friend list story and sent it to me as well (not many are aware that we’re dating, except for this friend)

I don’t want to dismiss this. I’m not upset, I’m just curious what would be the best approach since the source came from a third party.