A guy I know is also learning S&T and he started to struggle on the course the other day, especially with the driver. So I had him tee off with the driver using his normal swing, or the swing that was causing him fits, then another shot, but this time with a full "who cares, rip it" swing. The results were interesting.
His normal swing was shorter (more controlled) which was not a fault in and of itself, but, it started to become clear that as he took the club back his weight migrated onto his trail foot for which he started the downswing from there and the shot was kinda weak and low left. Then on his next swing, he took the club all the way to the top and hit a great drive. It dawned on me that while his weight drifted to his trail foot on the takeaway, as the club got to the top his weight moved back over his lead foot putting him in the optimal "stacked over the ball" at the top. Therefore on shorter swings, his weight was on his trail foot because his swing had not moved enough to the top which therefore would have promoted weight transfer back to his lead foot. Hogan seemed to make this same type of move, or his weight moved right on the takeaway but as the club reached the top his weight was clearly on his lead foot.
After going over this with the other guy, I tried it myself and found the full swing has a much better chance of getting stacked over the ball, and I have to be super careful on shorter swing shots to keep weight on my lead foot to make sure I'm not starting the downswing from my trail foot.
@GolfLivesMatter - Hogan had this going on with his hips, and it got him clearly moving onto his front side as he neared the top. I speak of it also in this Tailbone2Target Drill.
Nice analysis of your friend's issues! Just be careful not to get overzealous and go beyond your body's natural stopping point in the backswing, because that's when the breakdowns start to happen.