I only have a D300 so I'm only familiar with it. I believe the D2X uses the EN-EL4a battery. If I correctly understand what I read your friend will need tohave the MS-D10 adapter to go with his MB-D10 grip. With this combination he should be able to use his EN-EL4a or the 8 AA Ni-MH batteries you mentioned.
How is the higher power handled? We have to trust that the Nikon engineers have done their work correctly and that the grip and/or camera has the necessary power regulation circuitry to prevent damage! The Nikon information does indicate that 8 fps are possible when using multiple batteries. Wow!