The important issue here is, is it a "hardware failure" (broken heads, platters won't spin, bad controller, etc.), or is it a "logical problem" (corrupted directory or partition map)?
If it's a hardware problem, there's probably not much you can do to get the data -- the drive needs to be sent to data recovery experts, and the cost of recovery will be VERY expensive.
If it's a software/logical problem, you have a fairly-decent chance of recovering at least -some- of the data on the drive. To do this, you will need:
1. A "dock" in which to put the problem drive
2. Recovery software
3. A "scratch volume" to which you can recover the data
I suggest you (or her) get a USB/SATA docking station. They are cheap and easy to use:
http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias=aps&field-keywords=usb+sata+dock&x=0&y=0
For data recovery software, you might choose from:
- DataRescue3
- Stellar Phoenix Data Recovery
- Disk Drill
- Nice To Recover
These all work pretty much the same way. That is:
1. Download the app for free
2. Run it in "demo mode" to see if it can access the problem drive
3. Allows you to recover -ONE- file for test purposes
4. If it can see the drive and recover a file, it can probably do much more for you. Then you pay the registration (typically less than $100), get a code, enter the code, and let the software "go to work" on the problem drive.
5. The software really needs a "scratch drive" (empty drive) which serves as a receptacle as it scavenges the problem drive and reconstructs the files.
Suggestion:
If you've read this far, and decide to try data recovery on your own, be aware that even with data recovery software the problem drive may not "mount" on the desktop. In that case, the data recovery app may not be able to "see" the drive.
If that happens, there still remains an alternative. That is, to re-initialize the drive (yes, you read that correctly), and then "attack it" with the data recovery software.
Remember that re-initializing replaces the drive's -directory-, and normally does not erase the actual data that lies out on the sectors of the drive's surface platters. (IMPORTANT: DO NOT choose to "zero out" the drive -- this WILL erase the data on the drive's sectors. You DO NOT want to choose this option)
With a new, "fresh" directory, the Mac can now mount the drive, and even though it appears to be "empty", you can use the data recovery software to scavenge the drive and re-construct the data that's on it. You WILL lose all folder hierarchies and probably most of the file names as well. But you can get the actual data back, and that's what's most important.