Could there be any negative effects from using a nebulizer when it is not prescribed?

My son just spent the weekend over at his grandma's house. He left here with a cold and over the weekend it developed into bronchitis. The doctor told him just to drink plenty of fluids and it will go away on it's own. His grandmother hooked him up to his uncle's old nebulizer used for asthma. Is this harmful or won't it make a difference. My son is 6 years old