A fitness for duty examination is "job related" if it is "tailored to assess the employee's ability to carry out the essential functions of the job or to determine whether the employee poses a danger to himself or others due to his disability. There is a "business necessity" for a fitness for duty examination if "the need for the disability inquiry or medical examination is vital to the business." (Cal. Code Regs., title 2, section 11065(b)).
For example, in a situation where a professional driver passes out while driving or reports feeling dizzy, the employer will definitely have the right to send that employee to undergo fitness for duty examination in order to determine whether it is safe for him to continue driving, and what, if any, measures can and should be taken to minimize the risk of accident due to that employee's physical condition.
Generally, unless fitness for duty examination appears to be unreasonably invasive, it is in the employee's best interest to comply and undergo such an examination in order to help the employer to do what is necessary in order to accommodate that employee's disability or medical condition as required by ADA / FEHA or FMLA/CFRA.