Correcting improper form

Question
My 18yo over the winter has developed a bad habit of taking a step off the plate when pitching.  I believe this direct of her basic fundamentals as she was taught to slide her lead foot from the right side of the plate to center as she is raising her arms in preparation for delivery.  This will now in an illegal pitch everytime.  I am of the opinion she needs to keep her lead foot on the center of the mound plate and lift her toe in order to stop this flaw.   Your opinion on the matter is appreciated.

Answer
Karl,
I teach my girls to put the toe of their lead foot on the back of the rubber,and their push off foot with the arch just over the crest of the rubber, this will encourage them to keep their weight back and their lead toe on the rubber.
It's tough I know, and it takes 3 seconds to develop a bad habit and 3 weeks to fix it.
Work with her with this and encourage her to do it correctly, practice,practice,practice and above all be patient. Without patience, things will get worse.
Hope this helps!
Good Luck,
Coach Suthe.