I used to be able to optimise Mac OS to within an inch of it's life, but I've forgotten most of what I knew already.

Generally, Macs don't get slower and more crash-prone over time like Windows has a tendency to do - there is a lot less going on behind the scenes and most optimisations can be performed by carrying out some pruning in the system folder. As suggested already, play around with extentions manager - if you do anything that causes a problem and the machine won't boot, hold down the spacebar at boot to bring it up again, or hold down shift to turn off all extensions.

One common problem is too many fonts - if you have more than about a hundred fonts available, the systm will take longer to boot and each app will also launch more slowly. Check in System Folder>Fonts.

Another is that the virtual memory setting has been changed - In System Folder>Control Panels>Memory. Check it is turned on and that it is at least one meg higher than the real RAM installed in the box.

On the subject of memory, go to apple menu>about this computer and check how much memory it thinks is installed - maybe a SIMM/DIMM has worked loose.

Mac OS doesn't multitask too well - have a look in system folder>startup items to see if anything is launching at boot and sitting in the background slowing the system. If he has any pro graphics software installed you might see something like kodak color launcher - it's harmless.

If he is running anything prior to Mac OS 9, then it may be time to upgrade. I'm reluctant to suggest a complete reinstall as it really shouldn't be necessary. Mac OS X probably isn't the answer as his hardware probably isn't up to it.