Yes, you can do this.
Firstly, you need to create a new user for your friend and set the 'disk quota' for this user to 10GB (or whatever).
You can then add a new domain, or sub-domain if your friend doesn't want the expense of registering a domain, and specify the user you just created as the FTP user / CGI-runs-as user: for that domain.
Now your friend can log-in with FTP using his users credentials and he will only see the files related to his domain, not yours. Also, his disk usage will be limited to whatever you set the user's quota to.
If you need detailed instructions on any of these steps, just say and I will do my best.
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