The easiest way to do this is to publish to disk first (as I believe you have done). Make sure the folder name where you published is web compliant - preferably all lower case and no spaces. Here is an illustration of such folder on my desktop:
and here is a partial screenshot of the folder contents:
Note that it has an index.html file - this is important!
Next, connect to your hosting server with your preferred FTP software (I use Cyberduck). Make sure you can see the public_htm folder in the directory structure.
It may be that your FTP connection is configured to upload to your public_htm folder directly. If that is the case, you will see a file and folder structure similar to this:
This is essentially all the files and folders associated with your main site, So you can upload directly to this directory,
Upload the whole folder from your computer to your server - ensuring that you don’t inadvertently upload to a folder WITHIN the structure. It should appear as a folder on its own in the listing. Here is an example of how my folder appears in the directory listing:
Now, all your client has to do is enter the full URL of your domain adding the folder name at the end. e.g. https://yoursite.com/gfholding. Because the folder has an index html page inside it, that page will open by default. When your client has done with reviewing you site, you can go back to your FTP application and delete the folder from your server.