I would forget about the foot, work on thier hip turn. Should take care of itself.
If you put a heavy sandbag or weighted bucket next to the foot, makes it damn near impossible to turn.
DD's hitting coach puts a water bottle behind the foot so that if it turns (squishes) it will knock the botte over. They get a chances to fix it, them it's up downs every time after that (obviously, the up downs are for students who know better). I think having something happen when they squish helps them to feel/find the fix of driving the hip instead of the foot.