Probably has multiple laminated QR codes. That's not really the right way, or the best way. But it is a way that works.
With only one code, the restaurant can't know for sure who the payment was from. Though at these low volumes, the amount and/or the time would probably be adequate.
Even with a little more volume this method should still work fine, even if you get a second or third QR code to use.
Or the grill owner periodically generates a new address with which to receive coins.