Ask the doctor if a patient file exists, a billing statement, anything that supports this letter with no letterhead and a signature that claims to be a doctor.
Fire the employee for submitting a fraudulent document. Sick leave is paid leave and a fraud here is stealing from the employer.
and taxpayers as these are public funded positions
A state employee lying on the timesheet (and that's exactly what this is - sick leave for a protest) is a FELONY in my state.