You can set up partition with the free Windows 8 beta (it's quite stable, and there's a very easy way to turn off the stupid Metro tile UI and get a classic desktop) if you don't want to pay for windows 7. Then install Sleeping Dogs on that and delete the partition when you're done. But of course, you also need a dx10/11 GPU or it's a waste.
I hope so for him but because he's using Windows XP so I'm afraid he doesn't have a DX11 card.
You can run the game on a DX10 card fine, just without some DX11-exclusive effects and I don't think SD has them anyway (i.e. tessellation).
A lot of us were using DX10 GPUs on windows XP
They were quite common during Vista's life cycle and it wasn't unheard of for people to buy Vista PCs and then reformat with XP due to Vista's horrible quality. I myself had a DX10 laptop and used XP on it for four years. Also at the time the only game that actually made use of DX10 was Crysis which still looked good and ran better in DX9 mode, giving people even more incentive to stay on XP.