Which is why I love Seinfeld. So, you'd turn me in, Vamp? You Bastard.
I would first ask "How did it happen?". If it was something like my Friend just walking up to someone and killing said person, I'd most likely stay the hell away from him, in fear that I'd be next or in fear that he might give me the blame of the Death. If it was Self-Defense, then I'd tell my Friend to talk to the police. You cannot go to jail for Self-Defense.