An "internal server error" in all but a few cases means a server side problem (usually a misconfiguration of an .htaccess), but as the rest of us are clearly fine I've found it's then best to work "outwards".

Instead of visiting http://www.readandfindout.com, ask him to try visit a page he hasn't before, eg: http://www.readandfindout.com/rpg/messageboard/

If that doesn't work, try a different browser (firefox etc).

If that doesn't work, try a different computer on the same network (assuming he has access to one).

If that doesn't work, try a computer on another network.

Then we can hopefully track down whether it's a local, network, server or other problem.