The stabiliser does produce a downforce for the reason given by wizofoz. See photo of camber
here and below. The elevator is normally neutral but is deflected by the yoke to control the aircraft in pitch. If the new pitch attitude needs to be maintained the stab is trimmed (rather than the elevator) to minimise the drag that would be caused by the deflected elevator.
Your 757 friend was referring to the fact that most airliners are trimmed to the aft limit to reduce the amount of downforce required by the stab (because the weight/CofG is doing the work) thereby giving more net lift & hence more efficient flight.