In its response to a request for legal opinion, the FAA concluded that a pilot working for a part 135 certificate holder cannot perform work for that certificate holder, whether directly or indirectly through a related business or corporation owned by the certificate holder, during the pilot’s mandatory rest period.  Although the certificate holder is not required to monitor employee activity that takes place after release from work, such as in the case of work at a restaurant or some other business, including flying for another certificate holder, the FAA cautioned that both the flight crewmember and certificate holder would be in violation of FAR 91.13 if a flight crewmember flies when his lack of rest would endanger others.