Then it won't matter if when your drive/PC fails. Him having a backup on the same machine is almost as bad as not having one at all, IMO.
I have to disagree - Yes, I personally go for a waaay more paranoid backup approach, but just backing up to an external USB HDD (though with a "real" backup, not his manual drag-and-drop BS) puts someone a whole world of hurt better off than 99% of computer users.
If Grandma calls and says her HDD died and she hasn't "run that DVD backup thing" in a few months, well gee, sucks for you, granny! If, however, she calls and asks for help getting her nightly USB drive backup reinstalled to a new computer, hey, cool, she's lost almost nothing.
Now, sure, perhaps her computer got hit by lightning and toasted both. Perhaps her house got flooded and nothing electronic in it still works. Perhaps her PSU went bad and toasted every component in the machine (although even then, USB devices will often survive that). Perhaps she caught a cryptolocker-type virus that ate the backups as well. Sure, a single connected backup device has a lot of points of failure in common with the system drive itself. But in practice, it drops that "lose everything once every few years" down to "lose everything once in a lifetime".