You lose the NNID on the source 3ds, then you boot into hourglass9 (assuming you made a backup like you should of) then restore nand.bin, a week later you system transfer it back and he'll have a working 3ds and you'll have a hacked 3ds with no NNID, then you just create one
This only works o3ds/2ds to o3ds/2ds or n3ds to n3ds. If its o3ds/2ds to n3ds you'll have to call nintendo to get both working NNID's