I host all my websites from a single VPN, and am about to sell some limited space of my Linode to a friend to host their new blog. It'll simply be another entry in Apache, and then a dedicated folder to that website which will contain all the Wordpress files.
My issue is with security. If someone manages to compromise his Wordpress blog, will my other sites on the same box be compromised?
What is the easiest way to prevent or make an effort to prevent any of the files in his website directory or anyone that gains access there, getting access to any of my other folders or files?