Once the computer is turned off the memory is dumped. So if he boots into the recovery partitions or boots to an install cd how is a virus going to get back on the hard drive?
True, but capacitors will still hold charge even after shut down, and things like cache and memory is not always 100% cleared. Also, I guess it is quite possible to infect the firmware of a hard drive with a virus (however very unlikely). There are also boot sector viruses can that can stay with in the boot sector.
This should no longer be a problem once EFI is adobted because drives won't have boot sectors, it will boot from firmware memory off a controller instead, but that is theoretical since EFI has not fully caught on. Apple uses EFI and they actually put a very small partition on the disk for the EFI.
I am digressing though, so back on subject.
If you want to be 100% sure, wipe the partitions, write zeros to your drive, power it off and unplug it completely to clear all cache and memory (un plug it for like 10 minutes) and wipe out the boot sectors.
If your "virus" still exists after all of that, then it has found it's way into the firmware of one of your pieces of hardware. Then you gotta start flashing firmwares or replace parts.
#EDIT
Hmm, well I guess I should read whole threads before answering such things since it seems the questions have already been answered. I was too lazy and didn't want to read all the replies so I just hit reply after that last post I read.