The flight instructor reported that, he performed a preflight inspection with the pilot receiving instruction and did not observe any damage to the tail rotor blades before the accident flight. They subsequently departed to a nearby training area and completed several terrain flight approaches to a sandy wash area using standard approach profiles. After one hour of flight and about 15 approaches they returned to their departure airport. While performing a post-flight inspection, the flight instructor and the pilot both observed substantial damage to the tail rotor blades. According to the flight instructor, at no point during the flight did they feel any unusual vibrations with the helicopter, nor did they observe any debris within the landing area. The flight instructor reported no preaccident mechanical failures or malfunctions with the helicopter that would have precluded normal operation.