well, if you're going to set your friend up with a sub-domain - such as sub.yourdomain.com it's very easy. Create a new user - the one for your friend. then go into Domains and add a new fully hosted subdomain. Set the CGI to excute as his user. Wait for DNS settings to take effect.
Now your new user can log into FTP (and ssh if you want) and only have access to his sub-domain. You can if you want also set him up with a web-id so he has access to the panel and set it so that she can only configure settings for the sub-domain.
As far as the forum goes it's just a one click install.