The harddrive does this:
Keeps your saves
contains your dashboard
boot-files for the dashboard
uhhhh... a few more things? :P
Do you have a chip or not?
Getting any video or sound?
If you do not have any video at all I am suspecting something more serious than harddrive failure... :-\
If you have a chip try booting with an install disc such as SlaYers or Auto-Installer Deluxe.
The sound you're talking about is also bad, as you should hear slight HDD spinning and some fan noise...
Lets say that your xbox boots and gives video and your HDD is bust AND you have a chip. Then it will be piece of cake switching hdd... Depending on chip it will be from easy as h*ll to very easy...
Here are 2 links that can help you out with the hdd switching...
HDD with modchip
No modchip hdd switch
Good luck!
If you do not have any video please reply and describe the problem a little more, also ask your friend more specific what happened.
Edit:
Oh, if you have an Xecuter 3 or Xenium ICE or similar 4th gen modchip you will most likely have an HDD upgrade wizard in the modchip boot menu.