I host all my websites from a single Linode VPN, and am about to sell some limited space of my Linode to a friend to host their new blog (it's going to be a simple wordpress 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 (I'm thinking ahead - I haven't actually sold him the stuff yet) is with security. If someone manages to somehow hack his Wordpress blog (He might have a weak password or there could be another flaw (I'm going to try and make it pretty watertight though - I have full server control over his site, he doesn't have SSH access or anything)), will my main site be comprimised?
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?