When disposing of an old PC, there is really only one way to securely erase the information on the hard drive: You must destroy the magnetic platter inside. Use a T7 screwdriver to remove as many screws as you can access. You'll probably be able to remove the main circuit board from the enclosure.
Simply so, do magnets damage hard drives?
While you can damage hard disks with magnets, it takes a very powerful magnet to do so. As this article explains, any magnet that your brother was playing with won't be large enough to do any damage: It will take a very, very powerful magnet to affect the data inside the hard disk drive.
Can you ruin a computer with a magnet?
Yes, a magnet can destroy the hard drive inside a PC, but you need a much stronger magnet than the type found attached to a refrigerator magnet. Here's a video of a Toshiba notebook being destroyed by a magnet. The magnet that caused the destruction is off a fishtank cleaner.
Can a magnet mess up a computer?
Because of how hard drives work, magnets won't delete anything from your hard drive. While a magnet isn't going to wipe your hard drive, if you leave a powerful magnet directly on top of your hard drive there is a slight chance that it could cause damage to the hard drive itself while it's functioning.