Conscious of Consciousness

A Dhamma discussion based on the topic (paraphrased): "I was discussing with my mother about the impermanence of the five khandhas. She could see how each of them were impermanent, except for consciousness. What proof is there, for example, that there isn't some little man hiding behind, in the mind, and observing everything? How can one contemplate consciousness as impermanent, since it's consciousness that observes? Do you have any advice on how I could answer her Bhante?"