I see the old chestnut of Fenestron stall raising its ugly head again. I thought this had been thrashed out long ago. Extensive trials by Eurocopter and independently by Boscombe Down have shown that it is possible to regain control after so-called Fenestron stall. Nr Fairy is right to say that this phenomenon is now more accurately referred to as Loss of Tail Rotor Authority. The trials showed that even at extremely high rates of rotation it was possible to regain control by applying full opposite pedal and keeping it applied. The rate of rotation always (even if it sometimes took a while) reduced and eventually stopped. And these trials were conducted on the Gazelle with the older, less efficient feneston. The new generation fenestrons on newer Eurocopter products (EC 120, EC 130, EC 135, EC 155) all take advantage of developments to improve both the efficiency and the noise signature.
Unfortunately, I think that if this accident was caused by loss of tail rotor authority, then it was almost certainly avoidable.
PS I think the Boscombe Down report may be available to the public. Anyone know of any sources?