Has he really? There shouldn't be a reason why he couldn't get it, if he has. If he's seen them all, then he just goes to Rowan to get the National Dex. There must be a few he's missed.
Common slips in the dex are usually Wormadam, Combee, Lumineon, Chatot, the pixie trio, and Palkia / Dialga. Get your friend to check his Pokedex, and make sure he's truly seen them all. He may have to have beaten the Champion as well, in order to get it.