Commonly Croquis models would be nude, just because clothes make things harder to see. https://en.wikipedia.org/wiki/Croquis
But I guess it's not a rule that they have to be nude. XD
(Come to think of it, maybe she's also a nude art model at serious courses, while this one is perhaps a session for hobbyists, so she thinks he might have been to a real art school and dedicated her to memory so well that he can draw her naked body even without seeing it. He did draw good enough, given that criquis are usually pretty sketchy since they are to be finished in just a couple minutes.)