Simple. Leave some Breatharian literature at her house.

Now, sure, if someone told YOU that it was possible to live without eating anything at all, you would know at once they were one biscuit short of a packet. YOU wouldn't starve yourself slowly to death, all the while wracked with guilt that your inability to thrive on a Breatharian diet must somehow indicated some deep spiritual failing.

But then you aren't a quack.

Here is all is: http://www.breatharian.com/ I haven't laughed so much since Ken Livingstone lost the election. Check out the explanation of the five dimensional properties of a McDonald’s double-quarter-pounder with cheese meal under the FAQ.

DO YOU KNOW THE BASE FREQUENCY OF THE DOUBLE-QUARTER-POUNDER WITH CHEESE MEAL FROM MCDONALD AND DIET COKE?
Well, do you, punk?