I watched and listened to this video again and he's telling her to "snap the top hand". not stack the hands. She does know what he means when he's giving her these instructions.
Well, I will start and then others can give additional advice. I had to turn the sound off, so that I didn't have to listen to the instructor. My approach would be a lot different, but then, I don't know how often he has worked with her. He is using terms like "stack the hands" and "stay inside the ball" and I am not certain that she has been taught the terms. He sounded like he had just read a book on hitting.
She is too straight up and down and many times her shoulders seem to square to me.
We would be working the bottom half a lot and I would have her flexing the knees, almost into a "sit."
She stings her own hands at one point and she has to keep swinging, because they are using a machine. I wish that all instructors used a tee and soft toss. Then, you can stop and demonstrate what you are saying.