If you're really going from 30mph to zero, I might just say something vague, like, "You know, I've found myself feeling a little uncomfortable about our friendship. In this case it really is ME, not you. I hope you won't ask me to explain, and just trust that I mean it when I say I think you're a good friend and an excellent colleague. You've done nothing wrong, but right now I want to put an end to the "friend" part and keep the "respected colleague" part. I'm sorry this is awkward."
Or not. I'm just brainstorming.
