What's the legality of set-up fights?

Say I and an acquaintance met on private property just to fight, box, hit eachother fists only no weapons no crying foul and both participants willing, if the cops were to somehow catch wind of it would it be illegal?

I have a friend who says he needs to get some frustrations out and once to post an add on craigslist looking for someone to box with, thinking that there may be someone equally as willing as he is to get punched. I'm trying to discourage him but he seems determined, so i just want to be prepared with info in case it is illegal, but if it's legal he has my blessing.