Speed things up with a Solid State Disk (SSD), maybe?
Depends on your budget.
You might consider an external Hard Drive so that he can back up data to it.
Nice comfy desk chair is very important, especially one that can recline.
Uplighter for softer lighting so that your eyes don't suffer, it is very bad to look at any monitor in a darkened room for too long, I had a very freaky experience a few years back, I don't know how long I had been looking at my monitor in the dark but something distracted me and I looked away from my screen and all I could see was the following
so now I always have a secondary light source, an uplighter because the light is much more relaxing.
Wireless mouse and keyboard, I used to get fed up of the mouse cable getting tangled and the wires for both keyboard and mouse are never long enough so I bought a wireless keyboard and mouse, best purchase I have ever made.
Lots of paper and quality gel pens for jotting ideas down, sometimes development requires some kind of manual / written work, thrashing out code on a PC is all well and good but you have to have some plan of sorts.
Printer, a reasonable Laser printer if all he has is an inkjet, these start from around £80