I remembered a coworker of mine with that story; he was giving phone support to a guy, my coworker needed the guy to hit F12, the guy kept say: "Nothing is happening", after a while the guy finally told my coworker: "I don't know what is wrong, I keep pressing F, then 1 and 2 at the same time, but is not working".
That's when I learned that even when you think the guy on the other end of the phone is getting what you're saying, is best to make sure the message was clearly received so both of you can avoid wasting your time.
So maybe starting by saying: "how do you normally turn off your computer?", "how do you normally turn on your computer?" or "do you see a button that specifically says 'F12'?"; it goes a long way.