It depends on the situation. If there was intent to deceive then an illegal pitch could be called. And the situation also matters, since she was not on the mound. Generally speaking as an umpire we would declare play once the pitcher is on the rubber and batter is in the box. Since it would have happened before that i would have not called anything.
It is hard to give a ruling based on reading that. If you think about it in these terms it might make it simpler to understand. A batter can stake practice swings between pitches while not in the box or with one foot in the box. Same sort of situation. Also, the only real reason i could imagine a pitcher deliberately 'faking' a pitch would be to see the batter commit to a bunt. In that case it could be called illegal.