There is even a potential with this that the graphics card will need replacing. old S939 could have either PCIe or AGP... My old S939 board actually had both, so it was a really good board at the time.
S939 is old enough that it'll potentially be PCIe1.0, rather than the current 2.0 on all boards. Not all current boards have built-in graphics, so pay attention to this.
I would highly recommend a complete rebuild. Check the power supply for the CPU power. If it is only 4 pins, you'll need a new power supply, as many of the new AM3 socket boards, or LGA775 boards if you go to Intel will require the 8 pin CPU connector.
As said, S939 boards may have an old AGP graphics slot, which means the card won't work in *any* current motherboard.
Also, my old S939 board didn't have built in support for SATA, so even for just an "IDE mode" sata drive, i had to do the "press F6 to install drivers" if i wanted to run with a SATA system drive. This means your friend may have IDE hard disks... I would recommend getting a new HDD for this, as well...
Please suggest to him not to buy it bit by bit, as you won't know if anything is faulty until you put it together. This could potentially be months (buying one piece per paycheck?) which lowers your warranty duration. Suggest that he just put money aside for a while, and buy it all in one go. Several months down the line, current products will be cheaper, so he may even be able to get a better system.
Hope this helps.