The caller is right in that the energy a drafting car “gains” is “lost” somewhere else. But that lost energy isn’t from the fuel of the semi — it is from the drag behind the truck. And Ray is right that the truck benefits also — but not because the car is pushing the truck (an air cushion couldn’t be formed in front of the car as there is nothing to trap the air and the relative speed between the two vehicles is small).
A low pressure area is created behind the truck that tries to “drag” or pull the truck backward (the more square the back end, the more drag is created). The drafting car reduces the drag as it acts like a faring (making the rear of the truck more streamlined). This low pressure area “traps” the car (to some degree) and pulls it forward (thus helping the car) while the car reduces the drag on the truck which lessens the force pulling the truck backwards (benefiting the truck).
So the energy gained by the car would have been lost in drag and turbulence anyway — drafting essentially increases the efficiency of the truck.