There is a plugin called IQ Block Country (https://wordpress.org/plugins/iq-block-country/
). It allows you to block portions of your site to a country.
It does require downloading a database of IP addresses by country separately, but the instructions tell you where to get it.
I would challenge your client whether to go this route, or whether to block the entire site. If you could block the entire site, it could be done at the webserver level, instead of invoking a PHP process via WordPress. The concern here is performance.
Dealing with the blocking at the WordPress level has one other problem. This approach is essentially incompatible with page caching plugins, as caching plugins create static versions of your page. IQ Block Country plugin does attempt to discourage blocked pages from being cached, but many popular caching plugins do not honor this.