Seems like the old problem if you've never experienced it or heard of it all will be fine ...that is until someone goes down,only this week at my home airport Belfast City a fr 737 made an emergency landing when pax and cabin crew noticed a strong burning smell after takeoff aircraft returned safely,word is.... wait for it traces of ash were discovered in the engine.
I agree that the captains decision final regarding safety however on the flip side of the coin if we refused to fly for every little grievance that arisies we'd get nowhere.
Personally i dont think the pilot in question has anything to worry about as if in a tribuneral is can be proved that ash was predicted anywhere near the aircrafts flight path he will be vindicated.