The airplane owner/pilot of the tailwheel equipped airplane had not been flying often and hired the flight instructor to help her regain proficiency. This was their fourth flight together and they planned to perform touch-and-go practice takeoffs and landings. The flight instructor reported that, on the third landing, the airplane touched down at a slightly-lower than desired speed. The airplane then swerved left as it decreased in speed and he instructed her to maintain the runway centerline. The airplane subsequently ground-looped to the left and impacted a runway sign. The flight instructor stated that it was too late for him to take over control of the airplane. The airplane’s right wing was substantially damaged. The flight instructor reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation.