Q: Installing Leopard without install disc?

Not possible

Not legally. It can only be installed on one machine at a time, not multiple machines. But it is technically possible.

How to use FireWire target disk mode...

http://support.apple.com/kb/HT1661

http://en.wikipedia.org/wiki/Target_Disk_Mode

Put the G5 in T mode, use Carbon Copy Cloner or such to clone the G4's drive to the G5.

You do remember that this is not legal?

I'm assuming they want to just use the newer Mac once done & won't be running 2 copies.

I think, then, I might ask about that first insofar as they have both computers and not mention was made of wiping one of them. Just a friendly thought so you don't bet in trouble with the mods. You know how they are always "fishing" for violators.