I believe the contracts POS companies sign with VISA, MasterCard, and many others require the certify their hardware.
So I do not expect to see hardware agnostic POS providers.
I would love to be proven wrong.
Do they currently have a merchant account, or are they relying entirely on third party services?
I work in this space and there are providers who don’t require proprietary client devices (bring-your-own-iPad) but there is still proprietary hardware in the form of a small box (think MiniPC) that runs locally at the store so the iPads can still work while the location is offline (this happens far more often than I ever would’ve guessed).
Most POS systems, even if they let you use your own iPads, will still require hardware for taking credit card payments and printing receipts.