The flight instructor reported that during a helicopter "discovery flight" he was providing instruction to an airplane rated private pilot. The flight instructor further reported that prior to takeoff he told the pilot receiving instruction, "I am going to do the takeoff, and you can feel the controls." As soon as the helicopter entered a hover after takeoff, the pilot receiving instruction "put in strong left input" with the cyclic and would not stop after repeated instruction to let go of the flight controls. Subsequently, the helicopter's left main skid touched down and the helicopter rolled over and came to rest on its left side. The tail boom was substantially damaged. The flight instructor did not report any mechanical malfunctions or failures with the helicopter that would have precluded normal operation. The flight instructor reported on the National Transportation Safety Board Pilot/ Operator Aircraft Accident Report, within the owner/ operator safety recommendation section, he will not allow student pilots to touch the flight controls below 500 feet above the ground, until he is comfortable with the student.