No real world experience with this (1. Have never encountered that scenario 2. Don't use UPS anymore)
Internet info is ambiguous about what services along these lines UPS offers, and may depend on the particular franchise.
It looks like some (most, all?) UPS stores are like FedEx Office stores and have computers for rent which could be used to print the label for a charge. Unknown if store would allow printing of a UPS label gratis.
Seem to be indications that they can print online return labels given the "label code" (whatever that might be) but that may be only for return labels created through UPS directly. Don't know.
I doubt the tracking number would be enough for them to print from.
Best bet would be for buyer to put the PDF on a flashdrive and take it in and see. If they have computers for rent, those machines might be internet enabled and buyer might be able to print from the email you sent (ignoring the security implications of accessing their email account from a public machine)
Another option would be for you to snail mail the printed label with a 55¢ stamp in a plain envelope. Should get there in a couple of days.
You can fold the label in half to use a regular envelope as long as you fold it between printed areas and don't mess up the barcodes or addresses. If not a sticky label, you can include clear tape strips on an old stick label release liner or in/on wax paper for the buyer to use (who probably doesn't have tape either.) 
