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I think your engineer friend was refering to when the car was already up to operating temperature. in that case, its very unlikely that the heat itself will cost you power, the blower fan will.
if you wait untill your car is up to temperature then turn on the heat you can have your heat and drive it too, so to speak. once the car is up to operating temps (NOT warmup, NOT extending warmup, already warmed up) then the extra heat IS just waste heat that would only heat the radiator.
btw the radiator is not ALWAYS flowing engine coolant, there is only flow when the coolant is above the preset temperature control (usually a wax that melts at a specific temperature) valve and allows the flow to go through the radiator, its a paralell line on the coolant circuit.
though I belive many cars use the engine oil to heat the car, or used to at least
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