I think that if I were that customer, I'd be sorely tempted to post a dry-as-dust, utterly factual, scrupulously neutral account of what happened. That would be neither a review nor "negative or positive".

It would also seem professionally self-harming for the artist to sue, since I have a hard time believing that the court of public opinion would be on their side. They'd probably end up with a worse reputation (and so less work) than when they started.