Well, it is very convenient, I would say there is no risk. It would take a magnet many times more powerful that the weak one in the tip of a screwdriver to re-polarize an entire HDD plate through the HDD case.
A floppy drive as well perhaps but otherwise a computer is pretty much resiliant to magnetism if it's off. The BIOS can go funny if a powerful electro-magnetic charge is applied but no average magnetic screwdriver has that kind of power as it's disposal.
Yes, true. If he did something to mess up his harddrive, he did something else. The harddrive has a very strong magnetic head attached... Also, when you turn on the power, so the electrons flow about, weird things happens.