Best friend dissappointment

So...i went out last night with my two closest friends, both of whom are angaged. We were having a great night until i spooted one friend kissing a guy (not her fiance) on the dancefloor...she then dissappeared for like half an hour after that. I had to leave before i had a chance to confront her about it, and now she keeps sending me snapchats of her fiance with love hearts next to him.

What should i do? I am so unbelievable cross and let down by her actions, but i probably wont see her for weeks now, and i dont kmow if she knows i seen her kissing this random stranger in the club!?! HELP