Four matter most for a law firm: Personal information, Off-topic, conflict of interest under Rating Manipulation, and Harassment. None of them asks who was right. Three turn on what the review says. Conflict of interest turns on who wrote it. They all sit in Google's content policy. The general question, whether your firm's reviews have grounds for removal, has its own page.
Personal information. A review from the other side of a matter can name your client and describe her injuries. That posts a third party's full name and medical details, and Google's Personal information policy lists both. A review that names your attorney is a different thing. Google allows the name of a professional doing business under that name, and lawyers are on its list. It also allows a name that's part of the business's own name, so the name partners on the sign are covered too. There's more on personal information in Google reviews.
Off-topic. A review has to be based on an experience at or with the business. A one-star that's commentary on personal injury lawyers as a group, with no dealing with your firm in it, falls under Google's off-topic rule.
Conflict of interest. Google's Rating Manipulation policy covers reviews based on a conflict of interest, and it names former employment and industry competitors. A one-star from a paralegal who left last month, or from a firm that competes with yours for the same cases, fits that pattern. The other side of a matter isn't on Google's list, so in our read an opposing party's review isn't a conflict of interest on that fact alone. When we find a conflict of interest, the verdict is possible grounds, never strong. The worked examples are on the conflict-of-interest clause.
Harassment. A review that makes a specific threat of harm against your attorney or someone on staff, the kind that would make a reasonable person fear for their safety, falls under Google's Harassment policy. Anger about how a case ended doesn't reach it. There's more on harassment in Google reviews.
The other sections are in the Google review policy reference. The same content policies apply to your reply, too. Google checks an owner's reply against them before it posts, and asks you to edit one it doesn't approve (Google's help page on replying to reviews).