When faced with an opponent with whom you know, do you engage him in combat or do you concede?

I am faced with the possiblity of fighting someone I know, have known for a long time, and have too much love and respect for to fight voluntarily. It does not matter how legitimate of a fight this is going to be, but rest ured that it will be one to remember. How I am to remember it is my problem. Do I concede from the challenge and suffer what may come as a result but knowing that my opponent has feigned injury by my hand or do I stand my ground and suffer the truth that my opponent, my friend, will endure substantial injury as well as I be hated by those with whom my friend knows?