Even in a lesson, you would think the instructor would give the kid some high balls or low balls or balls he has to move for. To your example, he only taught the kid subtraction over and over again.
From the OP scenario, I actually figured the instructor was probably a friend or relative who knew how to play tennis but didn't know how to teach and probably wasn't paid. In which case, I applaud him for trying to help the kid out. However, if it was an outsider getting paid significant money, it certainly was a bad deal. (Even if the kid didn't practice, a good pro should have known the kid's abilities and not been "disappointed" that he lost so badly.)