EASA regulation AMC1 ORO.FC.230 does state that for the person conducting line checks "in the case of long haul operations where additional operating flight crew are carried, the person may fulfil the function of a cruise relief pilot and should not occupy either pilot’s seat during take-off, departure, initial cruise, descent, approach and landing."
That said, if they are exhausted then surely they should declare themselves as unfit to fly/fatigued.