I would say that there is no advantage gained by stacking fans.
Let me explain my logic.
If both fans are rated at the same CFM, the first fan is only going to push that volume of air.
The second fan can only push the volume of air supplied by the first fan, so the second fan is limited to the maximum volume that the first fan can push.
If your friend wants to make good use of the two fan set up, he should put the fans side by side. That would make them independent of each other and he would get a greater volume of airflow.....