It also is going to still be 85% full, and old. And any program he might use from that hard drive will still store data on that same drive. That means he is not creating any more room for his primary programs. He would just be adding a new drive that wouldn't be used much.
Then he is stuck with a new, great, drive and another old one with no room on it. Logic says, if he wants to slave it, then he should format it into a single, or double, partition drive and use it with his ZIP program to do backups only.
I do like the idea of putting the old drive into an external enclosure so he can turn it off when he isn't using it.
My primary concern is his comfort level. He's done a few things for me in the past, so that helps. I'll be sure to make this point to him, so he can consider it.