Construct runs great on my machine, which was bought in '98. It's a P3 600, 450MB of ram and a Geforce MX 420. And XP. Probably all he needs is a new graphics card and XP, which would be cheaper than a whole new computer.
Actually with anything that old you'd be better off buying a whole new rig (I don't mean to sound like a spoiled snob here, my computers are still bad too)
But it's a bit of a waste of money updating outdated technology bit-by-bit, because you either update the motherboard first and have to build the rest of it later on your new Mobo or you update the motherboard later in which case you've gone and spent money on something probably incompatible.