Technically if you both have the same type of console (both n3ds or both o3ds) then you could back up your cfw 3ds with a nand dump and sd backup, system transfer his 3ds to yours, hack the game, transfer back and then hack their console. The only thing with that is you would need to call Nintendo to unlink your nnid from their console, and you would have to wait a week between the two transfers.