First, is this really a problem that needs to be solved? Personally, his place sounds like my favorite kind of book shop. A lot of bookworms prefer wandering through dense forests of precariously-balanced piles of books. Is he getting those people, or is he getting people that are expecting Barnes & Noble?
If they really do need to be cataloged, then the next thing is to forget all about trying to inventory the entire thing. Instead, you're going to partition the collection into "easy to catalog" and "hard to catalog": pick a section of the barn and make this the organized area. Get a barcode scanner (https://www.newegg.com/Barcode-Scanner/SubCategory/ID-583) and throw together a quick API client that'll take an ISBN and display a title, author, edition, and picture. If it comes up correct, great: book goes into the cataloged section. If it doesn't, it goes somewhere else. Make it really simple, so that a single keystroke can accept that book into inventory.
Grocery stores have to regularly inventory everything on the shelves. I worked for an outfit once that wanted to do it all in-house, so we bought the commercial Telxon handheld wireless devices and I set about figuring out their software. Turned out that they just wanted to speak basic telnet to a server at a pre-configured IP address, so I put together a sloppy little telnet server interface and staff were able to count the entire store right on the devices in a few hours. That's way more complicated than what you'll need to do, so, y'know, your thing is doable. You'll have the added benefit of free online book databases and better hardware and easier-to-hack-together software.
Also might not be a bad idea to talk to your local librarian. They're book nerds too and he or she might have an actual library science degree. This would be right up their alley.