What happens if you boot up in safe mode ? (power off, power on holding shift)
Has he got any Login Items ? (Prefs -> Users & Groups)
Has he got a whole ton of crap on his desktop ? (I'm talking hundreds of files here, not a dozen... if so, try creating a folder and dumping it all in there)
Have you tried zapping the PRAM and resetting the SMC ? (suspect you might have done the latter with your talk of option/ctrl/shift).
Have you done an AHT (hardware test.... not sure if its built-in on older macs, disk might be needed....try holding D during startup)
Have you tried repairing permissions in Disk Utility ?
What sort of hard drive is in there ? In particular what RPM ? (Apple->About This Mac -> More Info -> System Report)
What sort of processor is in there ? (as above)
Anything interesting happening in Applications -> Utilities -> Activity Monitor ? (CPU, swap usage etc.)
Anything interesting in Applications -> Utilities -> Console ?
Are you sure it isn't just Spotlight re-indexing ? (click on magnifying glass in the menu bar, there will be an "indexing" message if it is)
If you're feeling techy and fancy digging deeper, hit
Control-Option-Command-Shift-Period and take a look at the resulting zip that it will output in /var/tmp after a couple of minutes (normally the finder will pop-up that folder in a window for you when done), that key chord will give you :
o A spindump of the system
o Several seconds of fs_usage ouput
o Several seconds of top output
o Data about kernel zones
o Status of loaded kernel extensions
o Resident memory usage of user processes
o All system logs, kernel logs, opendirectory log, windowserver log, and log of power management events
o A System Profiler report
o All spin and crash reports
o Disk usage information
o I/O Kit registry information
o Network status
o If a specific process is supplied as an argument: list of malloc-allocated buffers in the process's heap is collected
o If a specific process is supplied as an argument: data about unreferenced malloc buffers in the process's memory is collected
o If a specific process is supplied as an argument: data about the virtual memory regions allocated in the process
More RAM never did anyone any harm, although OS X is quite light on RAM on its own, so I suspect the underlying problem is elsewhere (although swap usage etc. will tell you about RAM usage).
My money's on it probably being a 5400rpm hard drive in there (some of the entry-level 2007/2008 era models had those). If so and he's adamant he wants to keep the 2008 going a bit longer, then perhaps try swapping for a SSD drive. Before I upgraded to a newer model, I had a 2008 model, and a decent SSD did help..... not as fast as SSD + new model with more modern processor .... but it did help prolong its life a little longer nonetheless.
Enough to keep you busy SpringHeeledJack ?
P.S. Useful OS X performance diagnosis
cheat-sheet here (and top-10 Dtrace Scripts for OS X
here) both from the senior performance architect at Netflix......before Netflix, Brendan was as Sun, and has also literarily written the book on systems performance..... as well as conducting the occasional somewhat unscientific test.....