Security is a great angle, as is the fact that OS X has BSD under the hood and offers a lot of the benefits of a unix/linux box along with a great UI....these are better arguements to make when he's thinking about a whole new system, not just a PC upgrade since a cost comparison with considerations for software replacement is going to weigh heavily against the mac. Plus, depending on what the box is used for/planned to be used for, he might be better off sticking with the OS that he has.