You can pay by scanning a QR code in the Bangkok Bank app. Almost all restaurants, even small family-run Thai restaurants, let you pay by scanning, even small amounts.
Otherwise, 7-Eleven is your change machine. Or take out 4900 baht.
Just to check: did you already have a 30 day extension to the tourist visa (i.e. 60+30+60), or did you only extend your visa with the Covid extension (i.e. 60+60)?
(I thought you had to apply for the tourist visa extension before you could apply for the Covid extension)
In my experience, it's only checked by the airline check-in staff at the departure airport, not by immigration. So if there's a problem he could quickly buy a cheap onward flight in the region.