THE PRIVATE PILOT HAD RENTED THE HELICOPTER FOR A LOCAL FLIGHT. AFTER PRACTICING SOME MANEUVERS ON THE AIRPORT, THE COMMERCIAL PILOT/PASSENGER ASSUMED THE CONTROLS AND EXECUTED A 180-DEG PRACTICE AUTOROTATION TO A TAXIWAY. THE M/R RPM DECREASED TO 380 RPM. ABOUT 30 FT AGL A POWER RECOVERY WAS ATTEMPTED, BUT THE SINK RATE WAS TOO HIGH. THE TAIL BOOM SUSTAINED A M/R BLADE STRIKE.