Understood. As I said, ''I realize that you don’t want to have the bat literally as vertical as possible.'' Was just saying that - for whatever reason and in her particular case - that the instruction to have the bat as vertical as possible at contact seemed to bring about a better swing path for her and produce more solid contact (less slice). It helped her to make contact ''on the up through the ball.'' In reality, she wasn't vertical at contact at all. It just 'seemed' that way to her. Thus, that 'vertical' cue just worked. She thinks she's vertical, but she's really just doing a better job of keeping hands higher than barrel and also scrunching the back torso (in other words, not swinging so level or down through).
IMO, the most important part of being a "teacher" is finding a way to help the student understand. It doesn't matter if it is hitting or calculus... As long as that cue works for her, keep using it. It may/may not work as well for another student.