Has it been confirmed that it IS the HDD that is the issue or has everyone excluded all other options already?
What OS are you even using? Age of the computer? How much free space on the drive? Fired up task manager in case there's some software running which hogs the cpu on regular intervals?
No, it's not necessarily confirmed. But really, if the entire computer locks up suddenly it couldn't be his CPU unless he's on a single-core without multithreading support. The next obvious culprit would be his GPU, but I doubt he'd be having normal FPS on a GPU that isn't capable of putting pixels on the screen. The next step would be RAM, but I have a hard time believing a RAM stick that takes longer than 500ms to access a bit of data would be functioning at all.
His hard drive seems like the most likely case. It's already the slowest component of any comupter, combined with the fact that a system would lock up when waiting for the hard drive to return data to the system (because it's malfunctioning). It probably is having massive seek times compared to normal, or has to process through corrupted data as someone stated above.