You should be able to use any manufactured video cards as long as they are based on the same chipsets, hence the dual 7800GT.
Maybe he needs to reinstall the nForce drivers (provided with the motherboard), then reboot the system. Uninstall any old Nvidia graphics drivers from the system and reboot the computer. Install the latest Nvidia graphics drivers (provided with the add-in cards or from the SLIZone Downloads Page on the Nvidia website). Reboot the system. Have you tried that yet?
We've already mentioned getting NEW DRIVERS. It allows mixed vendor support for NVIDIA SLI:
It's the ForceWare Release 80, Version: 81.98