can cool boiling water be used to water plants?

I don't see why not. Its not like you are giving it a steady "diet" of cooled boiled water. Besides, plants get like 95% or more of their needs from the soil and sunlight, so this water shouldn't hurt it.

well i say an experiment once... the boiling water can be use to plans they can still survive... well they use a mongo bean in that experiment. the other water was natural and the water that is boiled at a microwave... both the natural and the boiled water was successful the beans grows leaves and roots... while the microwave one didn't the explanation was that the microwave brakes down the minerals of the water... so they also said that this is what will happened if you heat your food in the microwave it brakes down the nutrients and minerals of the food.

I think your father in law was very right.Natural water with all its minerals is best for plants.No point in making the water free of all essentials required for the plant.Your father in law gave you sound advise.

I am not sure what you mean by "cool boiling water " do you mean warm water? or boiled water that is cooled? then in that case I think there is nothing wrong in using that to water plants.