Right here!
What version of Mac OS is he running? Tell him to click the Apple logo, top-left, and then click 'about this mac'. It'll say what version of Mac OS he's on. Then click the software update button when he's online, so that it can download any updates he needs.
Finally, you don't need antivirus software right now. There are no viruses out in the wild for Mac OS X right now (assuming he's on that). The firewall idea would be good though, and Mac OS X comes with one. Click Apple logo -> system preferences -> Sharing. Click the firewall tab, and then make sure it's started.