Glider flight instructor reported the student lost control of the aircraft during landing. Flight Instructor took control and landed safely.

Narrative

I was glider flight instructor giving transition training to a holder of a private pilot - glider certificate who was checking out on a new type of glider for him. He was used to flying a different glider (SGS 2-33) with less sensitive controls.He was landing from the front seat. Winds were gusting around 15 knots so we carried 5 knots extra airspeed on approach and landing. Upon touchdown the pilot began a severe pilot induced oscillation (PIO) which brought us airborne again. I took over the aircraft and after several oscillations I was able to land it without any injury to either of us.We took the glider out of service to be checked out by our A&P; who was only able this week to check it out; hence the delay in reporting this. We had not thought there was much damage to the aircraft; but found out now there is substantial damage due to the tail hitting the ground hard during the PIOs.Any CFI has to make the decision as to how much to let a student fly and when to take the aircraft from him. Possibly what I interpreted as simply gusty winds on final were really the pilot over-controlling. If I had interpreted it that way I would not have let him perform the landing and would have taken him up again for more training before letting him land this type of glider.