Wouldn't it be possible to see what standards the printer supports, like PostScript, or the HP equivalent, and seeing if there's a USB driver that supports those and use that?
Here's this reddit thread- but won't help you since they're using the network
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Pretty much anything that'll work with a PostScript driver should work. You can use a generic PostScript driver on most printers. If she's okay with monochrome, a used laser printer is the way to go.
I would also aim for a networked printer. You could even have the computer and printer on a crossover connection, if you wanted. This would avoid having to get her machine to work with USB, and would also avoid the hassle of trying to find a parallel port on a more modern printer.