My engaged best friend kissed me?

Anonymous
I've been best friends with this guy since starting college, which is about 2 years. We've always had a friend relationship, although when I first met him he flirted with me a lot. He met his current fiance about one 1/2 years ago and they are planning their wedding. I'm also best friends with his fiance and she asked me to be a bridesmaid, which I was super excited about. Yesterday at the end of a hike I took with my guy best friend he kissed me and I freaked out. I have a serious boyfriend and I was horrified that he would kiss me when that could end up ruining his friendship and engagement. What should I do? I don't know if I should tell his fiance or what...it's put me in a terrible situation :/
Updates
+1 y
Thank you to everyone so far. This whole thing took me by shock so I'm going to think it over tonight and do something about it tomorrow.
My engaged best friend kissed me?
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