A couple of observations from the video. It was interesting how quickly the disorientation developed once they were fully in the cloud. I think they stalled because there appeared to be buffeting before it all went pear shaped. The other observation is that at no time did the instructor stop trying to fly the glider. There was certainly a high level of anxiety but there didn't seem to be uncontrolled panic. Going back to the start however there is no way the instructor should have allowed the situation to get that far. You are PIC at all times even if you are not manipulating the controls.