Nope, it's all the same parts. Depending on the model he might have any one of a number of different types of RAM and depending on the age of the machine the HDD could be SCSI, ATA, or SATA. What model is it? What does it look like?
On PowerMacs replacing that jazz is very very easy. On iMacs they tend to make it a bit harder. RAM is usually easy but they tend to bury the hard disks, especially in the G4 iMac (iLamp). Those G4 iMacs also use 2 different types of RAM modules, a full size one buried inside, and a SODIMM (latop size) under an easily accessible panel on the bottom of the base.