Please correct me if I am wrong guys. Based on my understanding,
Amy is doing good - Good here is an adjective. Amy is doing how? Here how modifies doing and hence an adjective.
However if the sentence had read as 'Amy is good' - Then good here will be an adverb as the sentence now means - Amy is how/what? Here how modifies the verb is.
My First Blog on my GMAT Journey
Arise, Awake and Stop not till the goal is reached