Is it an old injury, something currently going on inside the foot or the result of the horses comformation. For conformation I would check the positions of the horses scapulas. He's a big guy but you need to get behind him and look over the top of his back up his spine and see how evenly the scapulas match up from side to side below the withers. I would expect that they don't match up very evenly. For ongoing a current and old set of radiographs might be helpful provided someone qualified can interpret them if you can't.
Videos are handy.
The next time you take videos get some footage of horse and rider moving away from the camera in a straight line, especially when riding down the rail. If the rider is not balanced, the videos do less damage to egos. You said she was an experienced rider. Keep her honest. At least check it yourself the next time you watch her ride that mount.