DURING THE LANDING ROLL THE REAR SEAT PILOT/PASSENGER ALLOWED THE TAIL WHEELED AIRCRAFT TO DIVERGE FROM THE CENTERLINE. THE PILOT IN COMMAND TOOK CONTROL OF THE AIRCRAFT IN AN EFFORT TO PREVENT A GROUND LOOP, BUT FAILED TO PREVENT THE AIRCRAFT FROM IMPACTING A FRANGIBLE TAXIWAY DIRECTION SIGN LOCATED AT THE NORTHWEST CORNER OF THE INTERSECTION OF TAXIWAY 'D' AND RUNWAY 25.