Well it all comes down to how casual of a user he is. If he's only surfing the web - pretty much browser security is all he'll need above the hardware itself. I'm sorry to say Safari and Firefox security is less than ideal compared to IE7. Anyone saying otherwise does not frequent bugtraq or the industry boards enough to say. But outside of the browser....
The things that will matter are: How easy is it to get the computer fixed (liability), How many years will your support last, How much upgrading can the computer handle, How many years are left on the OS life-cycle. This will have a lot to do with his location and whom he buys from.
I preffer any company that offers limitless replacement on all parts for 3+ years with full-paid shipping. Here in Hawaii, local support companies are nil - there were a couple but they must have went out of business.
I will not remark about anything regaurding what Windows was (like XP) or what OSX was (like panther) and I will keep my opinions unbiased
Vista has hefty requirements - but PCs also get better hardware cheap (not many $1000 macs sold with 8800 GTS 640MB video cards out there). But, it many years in the making (not just a bi-yearly update that costs money) and will be valid for many years to come. It sports many real features and industry standards Leopard will not have with regards to how it manages it's resources and protects itself. Improvements to an OS's core effect the whole user experience from network quality to file security and usability.
In my honest opinion, Leopard is just a little more icing and a lot of bug fixes to an existing OS. Few core OS features will be added and will have little effect on their underlying applications. A lot of these features can be emulated with third party software.
As a tiger user I would sway anyone I could from using Leopard to anything else so that we could keep the market share low. Internet marketers would not take notice and my system will remain as safe as it has for so long.
To sway them to using Mac I would give them a dollar-for-dollar offer that is better than a PC with a satisfaction guarantee. Don't say one OS is better than another - be considerate of the person's needs and how they will be met when you aren't around. Just don't make up stuff like half of the responses in this thread. We don't need another
fanboi scaring the sane people away.