Then just use a watch-only wallet app (Sentinel or Bluewallet) on a simple mobile device, even without internet and import his xpub address (only for deposits) to generate an address anytime he need to receive.

But transacting with Bitcoin without internet for a business is quite unusual and not recommended.
Nowadays a 4G connection (enough to have a simple mobile wallet) is almost everywhere available.
Tell the man to get one and don't complicate things too much.

Seems like that would still require an internet-connected node somehow. I'd love for him to use Lightning, but I wondered if on-chain would be easier for now? Maybe I'm wrong so please enlighten me if I am!

trevor

https://github.com/cryptoteun/awesome-lnbits

bitcoin

How can I help my Mennonite dairy farmer to use/accept Bitcoin?

Use Lnbits offline PoS
https://github.com/cryptoteun/awesome-lnbits