for that long, just about anything currently on the market should be fine.
Others are discussing hardware, I'll discuss the OS choice a bit.
A 64 bit OS might be worth considering, in the timespan you are dealing with they will probably become standard. He might be fine keeping the computer, but at some point he might need a software upgrade that requires it.
As much of a mac fan as I am, in 10-15 years, who the fuck knows what sort of processor they will be using. I'd be hesitant to recommend one for such a long service life due to their propensity to change processors more often than that. Scratch them off consideration entirely if they are even on your list.
Linux is a possibility, if all the software he needs(or equivalents) is available. This can work now.
Windows Vista... Some find good luck with it. If this needs to happen now and he needs or demands windows, research heavily the hardware and software that works well under Vista. Vista can be made to run well, but there are a lot of crap drivers and a lot of applications that really don't like the security features.
If you can delay the purchase, consider Windows 7. It's like Vista, only done right.
You could get any old thing and run XP, but looking that far into the future, you really should be looking at a 64 bit OS. Sure, any hardware you get now should run a 64 bit OS ok, but it would be best to plan from the start specifically for it.