Tee shot is hit to the right, comes to rest on the cart path (right edge of it.)

Player can't hit his ball because it's on the cart path, so he needs relief.

Player marks his ball and takes a drop at the nearest point of relief which is a couple of feet right of the cart path.

Before he hits his shot, a fellow competitor says his feet are still on the cart path so he has to redrop and take complete relief.

Player was not aware, I was not aware, I can't find it in the USGA rules either. Is there really a penalty for hitting a shot while your feet are on the cart path after the drop?