Every helicopter with a vertical fin will experience changes in the lift it produces at various speeds and the Fenestron equipped aircraft just have bigger fins than most - as Hot and Hi says, to offload the Fenestron in forward flight.
I think it less likely to have been a factor in this accident since he was already in a low speed situation when the yaw started and the main effect of the vertical fin would have been felt as he yawed to give himself a downwind component - a tendency to weathercock towards complete downwind would have been the result, not a stall.