Perhaps something like this should be in a policy handbook?
"Employees may not use passwords that are used with any service outside of the company."
Zero enforceability (but some companies would probably ask for a list of outside passwords I'm sure), but in the face of a direct policy violation, 100% fireable, and can help a company in terms of liability.
Of course, if they do, then firing might be the way to go. First time mistake, second time incompetence.