or he pushed it in until it cracked or something, they look like they can fit you know, exept for the part that the agp slot is farther back than pci splots.
Now on topic, any graphics card is compatible with any computer, brand does NOT matter. What matters is slot availability like many people have said here. Now most modern graphics cards are AGP 4x and greater. This means that you would have to have a modern AGP slot to support these cards. AGP slots are usually brown on the motherboard and sit farther back, and above all the PCI (white) slots.
More than likely, if his HP is new, it has an AGP slot (brown), and therefor will support a new modern graphics card. If he does not, he needs to buy a PCI graphics card, which will run alot slower than an AGP card, but might preform better than his current one. IF he has an older HP, chances are he has an ONBOARD graphics chip. which means its integrated into the motherboard and he cant take it off. Chances are if you dont see the monitor connection in a PCI slot (on the back base of the computer), then you have onboard graphics and it probably will not have an AGP slot.