In the case of Derdiarian v Felix Construction it was stated:
" If the intervening act is extraordinary and not foreseeable in the normal course of events or so far removed from the defendant�s conduct, it may break the chain of causation."
In your scenario it would be impossible for A to foresee this gust of wind and the consequences.
A is not responsible for the blindness.
Assuming the ambulance driver was driving in a proper manner and the gust of wind could not have been foreseen by an ambulance driver driving in a manner considered reasonable in all circumstances, then he is not responsible either.
Was the roof maintained or had the property owner been negligent to a degree that an ordinary property owner would consider the roof to be in an immediately dangerous condition? Could he be liable?