For 10.5, your friend is in luck--compatible ATI Radeon graphics drivers (including the PCIe X1900 G5 Series) are included as part of the Mac OS X 10.5 installation. You can verify this here:
The 10.5.8 update from Apple may have newer ATI drivers than the 10.5 DVD would have, of course.
The X1900 G5 Edition has support from 10.4.7 with the ATI drivers update needed from 10.4.7 through 10.4.11.
As Brewster Magneto stated, OS X has the ATI X1xxx drivers native from 10.5 onward.