If there's important information on those Zip drives, I'd really recommend getting it off of them right away (using a different computer if necessary). Those things have got to be reaching their end of life. I'm amazed they still work at all.
My suggestion: Set up a Dropbox account for your dad. Using a different computer if necessary, drag the files off the Zip disks into his Dropbox folder. If he is concerned about privacy, use a Mac to create an encrypted sparse bundle disk image in his Dropbox folder using the instructions found at
http://unvexed.blogspot.com/2011/06/how-to-keep-things-secure-in-your.html
(If he signs up with Dropbox at
http://db.tt/Me4yRjt I'll get a little referral storage-space bonus... ;-) ...and if he signs up with an .edu email address from his alma mater, he'll get 4GB instead of 2GB for free.)
This is a far slicker solution than using those creaking ancient Zip drives, and safer then relying on thumb drives that, in my sore experience, have a lifetime of a few years at best (and often much worse if they're pulled out at the wrong moment). Dropbox serves as an off-site backup solution for him, too. And with that encryption trick mentioned above, the security and privacy are as good as it gets, yet he can access his encrypted files on any Mac (and his unencrypted files on any computer, period).
By the way, an alternative to Dropbox is SugarSync. (Same deal, subscribe at
this link and I get a little storage bonus.) The encrypted-sparsebundle trick works there too, of course. You can even have both services working on the same computers. Plenty of space for your dad's spreadsheets!