The girl I am Dating kissed someone else

So I am Dating this girl and I wasn't home on new years and she got drunk with friends and then she kissed a guy. Apparently she doesn't remember it and only knows it because her friend told her and there's also a video of her doing it. I don't know what to do I feel betrayed and idk. Any advice is appreciated

Edit: I feel like I left out a few details. Originally I just needed advice and just a little push in the right direction but now there's so many different comments I was better of not posting lol. Just a joke I appreciate all of you for giving me advice.

The video shoved her friend kissing a guy then the friend walked away and she went in to kiss him as well. She put her hands around his neck and it looked like it was a pretty deep kiss. I think this is all that needed to be added