The student was doing the take-off with the instructor talking him through the sequence. As the aircraft became airborne a "thump" was felt. The right main wheel and inner strut assembly had become separated from the outer strut assembly and was hanging below the aircraft attached only by the brake hose. After emergency services were alerted and in position, the aircraft was landed by the instructor on the grass area beside the runway. Inspection of the aircraft showed that the upper torque link lugs, where the link attaches to the outer strut assembly, had fractured. The reason the lugs fractured was not determined.