This wouldn't work in the case I mentioned because they actually already had a very strict definition of when water should be hot and cold. If this were vague or unclear in their culture, you could lie and say "Oh yeah, because you believe X and Y, Z is true" - ignoring the ethical questions this brings up.

But in their case water being boiled was something you did when you were sick. This already had them boiling water for sick people, the sick would get better (to some extent confirming their belief), and they'd go back to cold dirty water.