The theory, as far as I understand it, is as much a matter of aerodynamic drag, as "discing" drag. Basically, the windmilling prop is being driven by the airflow, the force required to turn the prop has to come from somewhere, and that force is actually the extra drag, when compared to a stationary prop.
Just as an aside, when I flew a Dash7, we had to be very careful on the flare, as when the thrust levers were retarded to idle, the increase in drag could kill 10kts if you were still flying. That was 4 huge props that generated drag, enough to slow an 18 000kg aircraft very rapidly.
If the student doesnot understand, tell him to go and research it, and come back to you with the answer, otherwise, the book "Flight without Formula" by A.C.Kermode will probably have a good (clearer) explanation.