Re: The real question
As others have pointed out, the girlfriend seems to have taken it to the dump, rather than putting it out for collection by a lorry. Even then, it would probably have gotten compacted at some point, whether by static machine or a vehicle at the final dumping location.
"What are the chances of a laptop hard drive surviving that?" Maybe better than you would think.
Rubbish compactors don't crush, it's more like squash (as you say). As a student, I worked for the local council on the bin wagons and from my experience, domestic refuse is quite springy. It certainly bounces back when the lorry is emptied at the tip. Unless you get unlucky and a thing like a hard disk gets caught directly in the jaws of the press, there a good chance the hard disk just got heavily squashed between a lot of nice soft cushioning, stinky rubbish.
My bin lorry job was in the days before widespread recycling, when pretty much anything you put out at the kerbside would be collected and dumped in the ground. Certain items would warrant some special attention as they went in the back of the lorry. We would clear the back hopper by fully cycling the compactor hydraulics, then put the special item in the rear hopper all by itself. Cycling the compactor once more would indeed crush the item between hard metal plates. Flourescent tubes were fun when they went pop but nothing could beat a big CRT going boom and you could feel it in the cab.