I don't believe that any regulator has ever taken action against a qualified airman acting as a passenger and sitting in a passenger seat, for failing to act to counter a bad move by the lawful commander of the flight.
Where the airman-passenger happens to be sitting in a crew seat, I would expect the limit of his responsibility to be:
a) To pass advice as might be appropriate where a serious situation seems likely to develop; and
b) To act if, in extremis, the aircraft, its occupants or people on the ground, are placed in immediate danger as a result of actions or inactions of the commander.
2D