You made the right choice here. Don't take it too personally when he tries to tell everyone that you were a bitch and led him on and whatever. He put this on himself.
I was in a somewhat similar situation but I was Mark and my girlfriend and I had been dating for 3 years. My girlfriend and our "Kevin" weren't lifelong friends like you and Kevin were but we had been a close group for 4 years. Our main group consisted of myself, my girlfriend, and 2 other guys. My girlfriend was the type that always had guy friends so this was normal and definitely did not bother me in the slightest. She never led them on, never flirted with them, and was most definitely "one of the guys". We all knew this.
I always suspected that our Kevin had feelings for my girlfriend but never brought it up because I figured it was harmless and since we were all good friends, I didn't want to start any drama and ignored it. It all came to a head when we all went on vacation together but I had to leave earlier than my girlfriend and 2 other friends. I wasn't nervous at all. Maybe I was a little naive, but I really trusted my Kevin and thought that our friendship was more important than his crush. Turns out, he confessed his love for her after I had left and went a little psycho when my girlfriend told him that she wasn't into him and that he was a fucking asshole for doing this. The thing that stung was that he never talked to me about it before or after the fact, never apologized to myself or my girlfriend for his behavior (It was a lot sketchier than I'm putting on but it's too long of a story to type out), and not once admitted that he was at all in the wrong with the situation. It made me realize that this guy wasn't actually my friend but an opportunist just waiting for her to choose him over me or something like that. In the end, my girlfriend told him to go fuck himself and it broke apart the friend group. To this day he still messages her saying how good they'd be together. I don't talk to him anymore and I'm not even dating the girl. She still won't date him even after we've broken up.
Your friend Kevin wasn't a good friend. He was an opportunist.