You mean the discs don't mount on the desktop? First off, the discs from his Power Mac G5 won't support G4 systems, so that rules out using them, regardless (though they should still mount on the desktop). Also, his discs will be on DVD - does your G4 have a DVD-capable drive? (if you don't, DVD's are typically ejected automatically after a minute or two if placed into a non-DVD capable drive)
MacBook Pro (15-inch, Mid 2012) 2.3 GHz Core i7, 16GB, 256GB M4 SSD
iPhone 8 - 64GB S/G • Sound System Audio Engine A2 • Display UltraSharp U2412M 24"
Custom Built Gaming PC Kaby Lake Edition | 16GB DDR4 | GTX 1070 8G