If the student already has an account, could they possibly sign in an then enroll in the class then? They can use this link if they already have the canvas.instructure.com account: https://canvas.instructure.com/enroll/MDPRTL
Edit: I suppose they might not have an account already, but also trying "recover password" on one of the more likely ones seems like a reasonable step to try.
Edit2: I re-read your post, and I see that it says that after account creation and the student tries to log in, that's when they get an "email in use" error? That's a bit strange, but also: I know the login form asks for their email address, but I've only ever had luck signing in using the username instead.