Timely post; I just set this up for a client today.
When you created an account for him, it created a directory like so:
so let's say username created was jonsmith
his files now go to /home/jonsmith/
Now, on your domain, you want to remap a subdirectory lets say directorytomap to point to
to point to his space where he is uploading the files...
So at the web panel, under your account, Domains, Remap Sub-dir.
In the Fully-hosted URL to remap: pull down your domain. In the box afterward, type the subdomain you want your friend to have ftp access to.
Have your friend FTP under the new user. When he logs in, have him create your domain name as a folder (Example: yourdomain.com) Get into that folder and make the directory you said you wanted to remap (yourfolder). Upload the files into that folder.
If you go to:
http://www.yourdomain.com/remappedsubdomain you'll see the files he uploaded.
Full details about the issues are in the wiki (click here)
I'm working on several things as I write this, so if something didn't become clear, feel free to write me back here and I'll re-state it.