He's using DDR2 memory, so it has to be socket 775. Best bet for him to have a dual core, is the Pentium Ds, as I don't think motherboards 3 years old, would support the newer dual cores. Unless theres a BIOS update for them.
I was looking at the technical specifications sheet, and it did not mention anything except Pentium 4s. So it's most likely that you can't upgrade to a dual core processor.