You're getting lots of gut reactions from people here but I encourage you to think clearly and have a mature adult conversation with your partner.
Alcohol impairs judgement, and I would discourage you from listening to the people in this thread who think they know your fiance better than you do. Here's the questions I would be asking myself:
How drunk was she that night? Was it possible her grin had to do with the general fun of the evening?
Have you ever had a moment where you were partying hard and didn't regret something you did until you sobered up?
Does this guy who sent you the pictures have any vested interest in this? Is he into your fiancee or has he given any indication he doesn't approve of your relationship?
I ask because it seems fucked up for someone to call it being used over a kiss, I could see terminology like that being used over repeated behavior but it just smells a bit contrived or over the top to me. There are lots of guys out there who will prey on situations like this.
Does this incident surprise you? Does it seem in or out of character for your fiancee? I ask because in general, it's tough for women in the thirties who already have kis to find a partner. If it's out of character it may warrant giving her a chance.
Do I want to build a relationship based on "for better or worse"? Would you want to be forgiven if you made a drunken fuckup?
I list these questions because to me this sounds like your usual alcohol thing, girl gets drunk, girls gets attention, girl gets kiss. Is what she did okay? Absolutely not. It is a form of cheating.
But it wasn't a pre-planned date, it wasn't multiple incidents, and no P entered V. As far as things go this seems mild compared to serious cheating, and I'd given serious thought as to whether you want to upend your life in the way the other people on here are angrily encouraging you to do.
Good luck OP.