Well, unless for some reason they provided U.S. identification at the time they opened their account—and I don't know why they would have done that, or what that would have been—from the financial institution's point of view at that point in time, there's nothing there to suggest that they are U.S. persons.
All of the information they probably provided on the account to open the account is likely Canadian in source. That's usually things like a driver's licence and that sort of thing. It would be highly unlikely, I suspect, that there would be anything there to suggest that they're U.S. persons, and therefore that would be the end of that.