As my colleague mentioned, sounds like his browser is pre-populating the field. They would need to clear that or switch to a different browser to try and login. There is nothing in our software which would, on it's own, pre-populate login fields.
To answer your other question, you would need to login as the user to view the invoice (either through the ACP or as themselves through the login page). As an admin, you still are not able to view invoices from other users.