I haven't run Leopard on a Mac that old but I will offer a little advice: have your friend make sure that he has sufficient RAM installed for Leopard. I think that the minimum is 512MB... If you run the installer with insufficient RAM the Mac will lock-up when restarted and the DVD must be ejected using the following procedure:
1) Hold Control-Command-Option-Eject Button; this will shut down your computer.
2) Turn the power on with the power button and hold Command-Option-O-F -- this will boot you into open firmware.
3) Now type eject cd all Macs and wait until the CD pops out.
I know this only because a client tried to install Leopard on a G5 iMac with its stock allotment of 256MB RAM...