Awblain, Yes, but the aircraft still has a tail fin and horizontal stabiliser, which I would imagine would tend to oppose the tumbling motion the witness described, if the engines were already shut down at altitude. However, I'm certain no-one has done any research on this, for obvious reasons... so who knows?
IF the fenestron suddenly stopped doing it's job at low forward speed (and I'm certainly not saying it did but the fenestron duct seemed to have a lot of damage in the photos shown in the media), the effect would be sudden, rapid and increasing yaw rate.
The recognised "generic" method of dealing with this is to reduce torque and fly away so the fin can have more of an aerodynamic effect (dosen't work so well on some types). If this for some reason has insufficient effect, the only way to regain control is to attempt to get into autorotation. In extremis, the engines might have to be shut down completely.
If a very rapid yaw rate occurs, the result might well be divergences in roll and pitch too. Centripetal forces might mean that even with fuel in the tanks it may no longer reach the pickup pipes and the engines might flame out of their own accord.
But all this is supposition because detailed reports are yet to come. Hopefully remaining evidence in the wreckage will provide more answers.