Be prepared to respond to a bad online review from the client. Create a standard response along these lines
"The vast majority of our clients are happy with our work, and recognize the effort and commitment we have to helping them in every way possible. But some clients aren't happy, and that is often because they're unreasonable and don't follow advice or act in a way that advances their best interests. We won't comment on this situation specifically, other than to say we did our best in this case as we do in all cases, and wish our former client the best."
Unreasonable, toxic clients will not be swayed by this kind of response but we shouldn't care about them.
Reasonable people will understand what we're conveying, and they'll appreciate the low-key, and sensible response.