What is the best way to improve someone's golf game?
For most of us average golfers, it's the short game that needs the most work. If you are not getting up & down in 2 strokes from 50 yards out or less, 90% of the time, then you'll take more strokes off your score by working hard on this area than by working on getting off the tee or approach shots or putting.
Ask your playing partners what part of your game they think you 'waste' the most strokes on. If 3-4 say the same thing then start there.