Reviews are almost always opinions, as a matter of law. Defamation does not apply to opinions, even if the opinions are wrong or false or motivated by malicious intent. And you won't get anywhere trying to make a legal claim for false positive opinions about your competition.
A false statement not an opinion ("There was a roach in my salad") tend to be unprovable or he-said/she-said, and the reviewer is not the one who has to prove anything.
These sites invite consumers to post their photos of the exteriors, interiors, customers, food, products, etc. -- no actionable conduct there.
If you can afford the legal services to challenge this course of conduct incident by incident, most likely around 20 grand, have a skilled and experienced litigator make a detailed review of each post to determine whether each is actionable. Otherwise, you will simply have to outlast him. Consumer review sites will almost never prohibit posts in advance (exception where there has occurred provable "hate speech") by poster).
By the way, no point in suing if the offender does not have assets that can be collected against in the unlikely event that you succeed. Would a collectable judgment in your favor be at your best friend's expense because his assets are diminished and his support obligations are therefore reduced? Something to think about.
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