Sounds like he's planting the front foot way too early before the ball has been delivered and then also struggling to get a second movement going towards the line of the ball. ( if pitched wide outside off ). Sometimes he overcompensated and goes too far across as well.
Opening up the stance might help a bit, but in this case it sounds more like he needs to wait a lot longer before moving his front foot. This could be because he is falling over with his head in his stance.
Try standing still for as long as possible before the ball is delivered and also try and keep your head as close to your left shoulder as possible( presuming you are Right handed ). This should keep the head upright and prevent it from falling over and dragging the front foot across the stumps.