I would think that it would all depend on how much room he's willing to devote to that sort of thing. at the bare minimum he'd need to keep the core rulebook, beastiary book and pawn set in stock. if he's smart he'll also stock the beginner box, a cheap way to draw people in. I know it worked for me.
After that I think Rise of the runelords anniversary edition + pawnbook is always good to have around for when someone comes back after they have finished with their beginner box and are hooked. That way they can return to sandpoint, the town they helped defend in the beginner box and and jump back into the action going on in Varisia. APG, ultimate equipment, innersea world guide, and additional beastiaries are also good.
the local shop in my town doesnt stock any adventure path/modules that I've seen, and I'm okay with that. I still have to finish RotRL :)
best of luck to ya.
*edit
also I'd tell him to call his distributor and see what's selling, they probably have a tonne of suggestions. just tell him to make sure to take anything they have to say with a grain of salt, because it will probably be a salesmen on the other end of the phone lol
Considering he's doing 'Magic' I'd suggest trying out the card game as well - no studies or anything to go by but I can't help but imagine there is a large overlap between people who enjoy card games like magic and people who would enjoy the pathfinder card game.
And that's just another vector to get people interested in the 'full monty' experience.
*edit* not sure if your store would be up to this - but another way to possibly crack interest is to have a store copy of the card game or BB and let people check it out (or even give it a try if you have a play area) while they are in the store.
I've ended up buying some boardgames due to my FGS having a collection of games for 'loan'