He made his last 90 degree right turn in ground effect, using rudder
only. I didn't see much of the touchdown or rollout, as I was running
for life and limb. The glider was undamaged. The pilot, on the other
hand...

To my knowledge, he never flew another glider.

A turn at low speed with rudder only is an invitation for a spin.
At low altitude, it will usually end exactly the way we saw in this video,
with the glider spinning right into the ground.

If your collegue performed that last turn at very high speed the glider
wouldn't turn with rudder only.
If it was at low speed, and below 2 feet altitude, one of the wingtips
certainly touched first and it wasn't very pretty.

If he was above 2 feet, the result would have been some glider damage.