Its easy ask them to explain money to you
if they fail:-
        you already won
if they succeed:-
        they end up saying it is controlled by government and backed by gold.
       

Then ask what is gold why is it that valuable
if they fail:-
        you already won
if they succeed:-
        they end up saying it is precious metal can not be created or destroyed on earth.

Explain that is what bitcoin is -A Digital Gold- cannot be created or Destroyed