My father-in-law has an old Kubuki Super Speed ten speed. He'd like to upgrade the rear deraileur and cassette to some kind of Shimano STI or the like. I should mention that he uses a Durham elliptical front chainring that he's quite fond of (for reasons unbeknownst to me) so he wants the widest range possible on his rear cassette.

Any suggestions?

It would cost more money than the bike is worth to upgrade to current technology. Suggest to him to get a new bike.