Win Vista Home Premium 64bit OEM or Win 7 Home Premium 64bit OEM both around £75, unless you have connections. Can also be brought on ebuyer.
A very strong CPU for photo editing below £100. A mobo with integrated video, so that a graphics card doesn't need to be bought - which is fine for online gaming and photo editing, but won't be ideal for games from the last five years. Good 4GB set of low latency DDR3 RAM. Perhaps he will need two drives, as I read something from 2003 that said everything being on the same drive would slow down doing photo editing - don't know if this is still true. Also sounds like a 1TB drive would be more up his street. In which case this is the one: Samsung Spinpoint F3 1TB (£62). You probably don't even need a PSU that powerful for the components I've listed, and your choice of graphics cards that could be added would be limited by this PSU. A nice case. A standard SATA DVD-RW drive.
Consider paying the extra £25 to get a 500w PSU at the start so you have the power should you wish to add a gaming card or other components later. And if you do get a graphics card, strongly consider an nVidia card since some Adobe apps can use the graphics card to add the CPU and speed up processing.