While practicing low level hovering during a dual training flight, the student, who had accumulated two hours total flight time, was unable to maintain directional control. The flight instructor therefore took control of the helicopter and applied control inputs to arrest the student induced movement to the right and to the rear. As the helicopter began to move forward and to the left, the flight instructor inadvertently allowed the left skid to come in contact with the terrain. The contact with the terrain resulted in a dynamic rollover, with the helicopter coming to rest on its left side. According to the instructor, there was no indication of a mechanical malfunction.