Well I will answer this from the opposite side, I think if my best friend were to wish for something bigger than himself, that bettered everyone's lives, I'd be willing to allow him to kill me. I think the same could be said in reverse. As long as the wish isn't a selfish wish and was something that allowed everyone on the planet to benefit from, I think my friend would understand and accept a knife wound to the chest. Anything below that though, I don't think I could agree with. Real friends are just too valuable.