The flight instructor reported that the private pilot, who had not flown for a long time, was receiving instruction to become current. After accomplishing flight maneuver training, they returned to the airport for the pilot to practice landings. During the flare for the third landing, with the private pilot on the controls, the left wing was pushed downward and struck the ground when the airplane encountered a wind gust. The pilot subsequently landed and taxied off the runway. The airplane sustained substantial damage to the left wing rear spar. The flight instructor added that, if he had positioned his hands on or very near the controls during the landing, the accident could have been prevented. The flight instructor reported that there were no mechanical anomalies with the airplane or engine that would have precluded normal operation.