It's important that the rider learns to self correct too. During the lesson tell this individual that if he/she looks down and can see one's toes than the foot is too far forward. Another idea is to look at the stirrup length. Is this happening at sitting trot mostly or posting? Again, lunge work without stirrups like others have said-taking into consideration safety-helps. One exercise is to take both legs out from the saddle, put them back and down and this can be done at the walk. This opens up the hip. Explaining a little bit about biomechanics to the rider might help. For example, when the rider is first learning posting trot, we tend to say "up" and "down", this is actually not correct. Another thing, check out the saddle. Hope some of this helps.