In general, for a review to be considered defamatory, it needs to be a false (dishonest) review of the customer’s experience, causing reputational harm. In the case that it’s true, it’s important to note that due to their nature, statements of opinions aren’t considered false as they’re subjective to the speaker and are, as such, non-defamatory.
For Google business reviews, there’s an additional clause. Defamatory reviews need to be made about an individual, a non-profit organisation or a corporation with less than 10 employees.