The insurance may forbid "instruction" given in the aircraft.? That could end the legal way, assuming he is even a MEI.

I was asked a long time ago to fly right seat on a Cheyenne 400LS for part 135 charters. I actually took a 135 checkride with the FAA for this. The PIC sat in the left, I sat in the right, and the FAA sat in the forward most passenger seat to observe. We took turns flying on the checkride. The aircraft did not require two pilots, but the company and passengers wanted two. I logged my time as SIC and even some PIC on part 91 flights that I flew at the controls. No one ever questioned my time. I did keep a copy of the 135 FAA checkride in my logbook for evidence.