Try to set his Reader to default Settings and then try to authorize again.
This error is seen while trying to open a book that is authorized for a user different than the user currently logged in.
To correct this error, please try to log in with the Adobe ID with which the content was fulfilled.
It's possible that some of the books were copied onto my device by my sister-in-law, but they were readable then, what would cause them to become unreadable now?
It would most likely be that they were authorized with a different Adobe ID earlier or they were authorized using an Adobe ID which has been joined with another Vendor ID/Adobe ID.