Moving laterally off of the ball, or “swaying” can be a real killer to an efficient move. As weight moves outside of the back foot, it makes it nearly impossible to get back to the ball, without throwing one’s hands and arms at the ball. This has a negative effect on the shape of the swing (plane), as well as on the speed. It causes the swing to bottom out at the wrong point of the arc, usually resulting in fat shots, with the occasional skull.
Getting your student to set up with more weight on the inside of the back foot, and maintaining it there through the completion of the backswing, will allow for a shift of the weight to the front foot, with arms following instead of leading. Consistency factor goes up, up, up!