your friends are correct, actually when your heater is on it opens the heater hose which makes the water pump turn with less friction, also cools the motor down which doesn't really help to save gas, but a hotter motor burns fuel better(not more, just more complete) if you car is overheating you can turn the heater on which acts like an extra radiator, if you turn on the defrosters on 80's and newer vehicles the air conditioner comes on to help defrost the windows, which will burn more gas, a/c allways burns more gas as runs off pulley's, some newer benz and bmw air conditioners run off elec motor not hooked to pullys which doesn't burn more gas, also hybrid vehicles have this type a/c(elec motors) sorry so long but i'm a car guy.
Good question, never thought about it.
I just recently heard that neither heating nor cooling makes that much of a difference. It effects cooling but it's so low that it does not make that big of a difference. Hope that helps.
My friend said that too. He said only the Ac uses gas and running the heater actually helps but not much. He explained it somehow but I don't remember.