Are your Leopard discs the OEM ones that came with your MacBook? If so, they will be specific to that machine, and won't work on the G5.
It may be possible to install Leopard onto your Wife's G5, by linking the two machines together with a Firewire cable. You restart the G5 in 'Firewire Target Disc Mode', then select the HD on it from the menu that the Leopard Installer programme gives you. Alternatively, using the Firewire TDM, you could use Carbon Copy Cloner to create an exact copy of your MB HD, including the OS. Another way would be to use an external HD caddy, install the OS on that, then put it into the G5.
Buy a second HD for the G5. Install Tiger on that. Link together using FTDM. Install Leopard on it from MacBook.
Maybe. Good luck!
The G5 can take 2 HDs. I strongly recommend using CCC to produce a back-up copy of the G5's HD, in case it all goes wrong. Burn all pics and music etc to DVDs/CDs anyway.
Alternatively just have a root around for a Leopard retail DVD on Ebay extc. I might even have one
Bear in mind there could be all sorts of reasons why this might not work. You may need to get a copy of Leopard that works with PowerPC, as the G5 uses that type of processor, not the newer Intel ones.
I do know that a Mac Mini will run off an external HD, so it's possible the G5 will allow the same.