|If you can afford to spend a few dollars, I would suggest an inexpensive USB adapter, and install it externally. That's the easiest and safest way. The following is an example of a USB cable adapter:|
Hooking it up internally is not as easy. If you decide to do it that way, I suggest connecting it to the second cable/controller if there is one. If there is already a device connected to that cable, make sure that the drive that you are installing is not plugged (master/Slave) the same as that drive. For example: If the drive that is already connected is plugged 'Master', plug the new drive 'Slave'. If the drive is plugged 'Cable Select', then you should also plug the drive that you are adding 'Cable Select'. In other words, both drives can be plugged cable select, but only one can be plugged Master or Slave.
If there is only one cable/controller, then the current C: drive will normally be plugged Master or Cable Select. If there is a second drive you will have to disconnect that drive and replace it with yours. Plug your drive the same as the one that you are removing.
If you are not familiar with the hard drive jumpers, the following might help:
(edit) I suggest that you save whatever files your friend needs from both drives. She should have backups anyway.