In all likelihood, nothing happened to his hard drive. I've been winning bar bets with this one for decades ... Back in the early/mid 80s, a friend and I were tasked with bulk erasing a couple dozen Seagate ST225 20 meg MFM drives. Having recently acquired a couple of rare earth magnets (industrial surplus, as re-sold by the late, lamented Haltec in Mountain View), we figured we could wipe 'em across the drives and be done with it. But first lunch, at Fred's on Middlefield Road.
For those of you who didn't grow up in the wilds of Silly Con Valley, Fred's was (and is!) a dive bar[0]. Heavy emphasis on Dive. Home away from home kind of place, if you're into that kind of thing. A good place for planning destructive testing of all kinds. So naturally, we decided that we'd pull a drive out of the computer, but leave it plugged in, turn the machine back on and wipe the magnets over it "to watch the computer lose it's tiny little mind". Which we did.
To our surprise, nothing happened. The drive trundled on, ignoring us and our magnets. So we found a bulk eraser for tapes and the like and tried that. Still nothing happened. I wrote a simple "walking ones" program to run over them instead.
I've done this with increasingly strong magnets, and progressively denser HDDs over the years and have never, not once, lost a single byte of data EXCEPT mechanical data loss when the drive head gets bent into the running platter (the traditional meaning of a HDD head crash).
Don't take my word for it, try it for yourself. Or read this account of somebody else trying it and getting similar results.
"It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so." —Samuel Langhorne Clemens
[0] If you pointy-clicky around in the go ogle link, you can get a look at the interior. Might not look like much, but a LOT of what happened in the proto-SillyConValley started in there ...