You can instruct him to turn off autosync in his library client settings. If you really don't trust him you can disable authentication.
you can do this yes.. however, if you read my post in link above you will find by turning off auto sync those clients can not 5 star, or change tags (which can be a downfall) and above all new files that you the server add to your library will not auto update on the clients. they will be forced, and YOU will be forced in your home network to walk to every (or teamviewer in) to every client you have, and hit file/library and then reload the server on the clients.
In our scenario this leads to 6 PC/laptops on a daily basis that would have to be manually refreshed to see the new files that have been added to library. which is not something we want to do. So our only feasible option is to leave authentication on, and leave all pc's in theaterview while disabling delete option, and making sure the library is not available outside home network.
It's what we here consider... a catch 22.