Get him to read the owner's manual.
From a Kawasaki manual:
- After the engine starts, move the throtle lever on the quipment to halfway between FAST and SLOW. To warm up an engine, run it for 3 to 5 minutes with the throttle lever in the same load position (halfway) before putting the equipment under load.
Stopping the engine
- Move the throttle lever to the SLOW position. Keep running at the idle speed for about one minute.
I always go by the book...