"Just buy a few different sizes and bring them with you. 2 pair, one for her to use, and the other as a gift for her."
that's another approach, very fair to them and easy in terms of prep!
but though feet fet not nearly as niche or strange a request as many guys assume, not sure if i'd want to introduce that request to someone so suddenly unless money to burn on the chance of a few average sessions. if you can vet that w. the provider by phone or email first, that can make the difference btwn an obligatory session where they'd wear that, or a bombass session w. someone enthusiastic and or versed in kink. kink w. an indifferent provider defeats the purpose of kink (unless you like the humiliation aspect).