Not taking sides one way or the other on the underlying issue here, however, technically Defamation is only a criminal offense in 17 U.S. states. And even in those states, it can only be criminally prosecuted in narrow cases (generally involving malicious and false accusations of sexual immorality) and rarely is ever actually prosecuted. In the rest of the U.S., Defamation is a matter for civil litigation.
If this restaurant were in the U.S. it would probably have a good case for a tortious interference lawsuit provided it could prove the individual conspired with other people to leave reviews, knowing they'd never eaten there, and with the objective of interfering with the commercial success of the business.