If they can prove they lost business because of his untrue review they should have a case and should rightfully sue for damages for calling their business "scam

If they can prove they lost business because of his untrue review they should have a case and should rightfully sue for damages for calling their business "scam". His defense should also prove that it is in fact a scam business otherwise it's not an opinion - this can be defamatory. Anyway it's only a civil case, no precedent so far.