I've already tried running my 5870 & my old 260 with the hacked drivers and it does work, but its not worth the hastle. Theres too much conflict between drivers and its unstable....... Plus theres a problem with over heating, my 5870 went from 37 idle to 50 after all the hassle of setting both cards up!
Ohhh I know the hassle of running two cards. But you don't need a 260 for physics. He could always just get a lower end Nvidia card, like a GTS250. But still it will generate heat that will raise to the card above.