doodler85 what you could do is use removable hard drive bays so the drives could be removed and stored in a firesafe easily I did that for a client recently worked out real nice.
firesafes don't work for harddrives. They are meant to protect paper documents, which will not catch on fire if the safe heats up to 300 degrees internal temperature. Your data, on the other hand, will be ruined.
Also, where would he put those removable drive bays? its not going to fit in his laptop and I doubt the G5 will work for it.
Besides which, removable drive bays are very expensive, I know, I looked into them for my own backups. It will just cost more than using the existing towers + internal drives... it is an option if he doesn't mind the extra cost. Also costing more then internals (but not as much as removables) are external drives, again, its just a matter of costing more, requiring a big ugly USB splitter, and the dramatic reduction in lifespan on externals (from what I have observed... inadequate cooling, crappy PSU, and lots of vibrations add up badly). an external drive just costs about 20$ more per drive than an internal one. But all of them are doable if thats what he wants.
I'm going to have to check with my friend to see if he has the space to accommodate two towers... I could always use one tower hooked up to the network that does nightly backups to a smaller box like Computers_Plus mentioned long ago, right? See any major drawbacks with that idea? If there was some emergency he could always grab the small box and run out with it under his arm in a very dramatic fashion (which would probably work better than my 60 lb CoolerMaster case, I imagine).
Only issue with that box is that it costs more money then "an old computer i have lying around"... that and it cannot run its own OS so no ZFS, will have to use whatever OS his computer has, its just a box for drives, so he will be plugging this to his G5 and have the G5 format it and control it as if they are regular drives...
I see no problem with it, heck, you could forgo the server and just have 2 such boxes... one as main storage and one as backup. Both can be plugged into the G5.
You could even leave the backup one at a friend / family member and only update it monthly. That way in case of theft you don't lose it. (burglars will take any electronics)
This certainly saves you having to learn how to manage a FreeBSD or Solaris/Illumos server.
I don't know about sources for info about FreeBSD, but excellent solaris info can be found here:
http://www.genunix.org/
You would want CIFS sharing for windows... but he is using a mac so I don't remember what it is that he needs instead. I assume illumos is the same since its basically a different distro of osol.but I haven't gotten around to it, yet.
It does have a learning curve, so really, only bother with it if you want the benefits of ZFS and am willing to put the effort for them.
If not, either use said box, or for a server, use FreeNAS (which is REALLY simple to use and has its own documentation)