Ball too far back in stance. Backswing too long, across the line.
If you pause video at around 0:05, past impact and not to waist high,
he is up on his toes literally jumping out of his shoes.
So maybe work to make sure, per Brian, that up to 7 or 6 iron club leading
edge at center of stance measured inside the heels and the back of the ball
slightly in front of that line.
Have him hit some 1/2 or 3/4 swing shots with his feet planted in the ground and
finish on the left foot toward the middle or heel and not the toe.
My 2 cents.
