Tailfin, fenerstron and SAS all had a hand in Fenestron Stall in a Gazelle in which I was a passenger. We departed Shawbury in a 20kt wind and promptly had a generator failure, we therefore returned to the landing point and having lost the generator, we had also lost SAS/Heading Hold. This made the pedal inputs much more responsive in the hover. As we carried out a lookout turn left prior to taxiing back, we passed through the downwind and the wind caught the tail fin. The rate of yaw increase caught the pilot out and he went to full right pedal. We then continued to spin several times and thought we had a tail rotor failure.I could see his right pedal was fully forward and totally ineffective.His immediate reaction was to gain a few feet clear of the ground but as he applied collective we started to pitch nose up and he over-corrected with nose down, by this stage we had probably gone through 6 revolutions, he couldn't bring himself to release the right pedal and unstall the fenestron (partly because we had never heard of fenestron stall, let alone discussed a solution). He then put down the collective and accepted the ensuing hard landing. He did keep us as level as possible and the aircraft was a Cat3. The fenestron was fine. I am certain Fenestron Stall exists, even though the Aerospatiale test pilots were not able to re-create that situation. In any case, they would have been briefed and ready to respond....not quite the same in my opinion.