I'd deploy rational thinking and unwavering efficiency. Disposing of the body would be the first and most important step. Nothing a handful of industrial plastic bags, a 2L container of bleach, a sharp blade and a shovel won't solve. Then I'd take him to a bar and ask him an assortment of question, the answers to which would determine weather I break his nose or pat him on the back. Either way he is my best friend and he will have to live the rest of his life with the knowledge and memory of what he did. The last words I'd say about it would be that if he ever has the need to talk about it, he should call me and no one else and I will never speak of that day again, unless he does. A person only has ONE best friend and to me that title commands unquestionable loyalty and trust.
captcha: no brainer
EDIT: Made the post, then read all the comments and DAMN there are some interesting cases. I specifically mean those people who would turn their best friend in without even asking questions and the few that were confused as to who or what we are talking about here... We are not discussing friends or bros or someone you met while in the pub. This is your BEST friend, someone you've known for years, who has shared in your triumphs and failures and stuck by you when you most needed it... How is it even possible to mistake or even have doubts about someone like that...? Even worse, people who would turn their best friend over because of morale or ethics. To me that just means that there is no one in your life that has been important enough to you to transcend law, government and all that subjective stuff, which is sad and is completely on YOU for not being worthy of that kind of relationship.
I've known my best friend for 15 years. We've stuck by one another through violence, sickness, relationships, growing up and changing, different jobs and university... I put a man in the hospital to protect a friend he didn't like but it was him that posted my bail and not the one I protected. Because of all these things I know him to the bone. I know how he thinks, why he does what he does and the same applies to him.
Lastly, for those who hold murder in such high disregard. I don't blame you, there are places in this world that would have you believe deeply in morality, law and government systems. Sadly, there are those other places that are on the exact opposite side of the spectrum and failing to understand that is terrible in itself. I need to see the world in shades of grey to live where I live and because of that I've understood that when it comes to people's ugly side murder is actually not that high on the list of atrocities.