Off-topic. A review has to be about an experience at your practice. A one-star that's all about the shared parking garage and the building's management, with no appointment in it, falls under this policy.
Personal information. For a dental practice this one cuts both ways. A review that gives your hygienist's full name falls under it. A review that names you usually doesn't. Google allows the name of a professional who does business under that name, and doctors are the first example on its list. There's more on personal information in Google reviews, including both carve-outs.
Harassment. A review that names someone on your staff and threatens them falls under Google's Harassment policy. Anger alone doesn't.
Conflict of interest. Google's Rating Manipulation policy covers reviews based on a conflict of interest, and it names former employment. A one-star from someone who left your front desk three weeks earlier fits that pattern. Google's Misrepresentation section repeats the conflict-of-interest rule in shorter form, so two sections apply.
Fake Engagement. This covers reviews that aren't based on a real experience, and it's where "they were never a patient" belongs. But we don't prepare an appeal on "never a patient" alone. So the appeal argues Fake Engagement or Off-topic and points to what your records show. That's as far as you can go. You can't know who wrote the review, and an honest appeal doesn't pretend to.