German employment law
As I stated before, the German employment law is to blame in the fist place. The FO had a motivation to conceal his health issues from his employer. He did it on purpose AND was aided by German employment law.
Under German employment law it is the responsibility of an employee to inform an employer if they were deemed unfit to work. An employer(DLH/GWI in this case) do not have the right to ask for medical information from any employee. It is their responsibility to tell their superior, to tell their employer if they are sick. Even doctors could not step in as the data would be protected.