When she took the drive and formatted it on her Mac, she made it HFS+. A drive formatted that way will show in some areas on a PC but it won't give you access to the data.
Typically you can do a reformat on the disk and it will be seen on your PC again...but remember...doing a reformat will delete anything that is currently on there.
Here's a quick walkthrough on how to do the format:
http://www.ehow.com/how_4768949_format-hard-drive-windows-xp.html
(The steps are for Windows XP, however it's the exact same steps for Vista or 7...the only difference is you're going to right click on Computer instead of My Computer.)
If you run into any snags or have any questions, reply back and I'll do what I can to help you.
Best of luck!
To all users: If your data is stored ONLY on an external hard drive, then it is NOT a backup. A BACKUP is having the data stored in TWO or MORE locations. If it is only in ONE location, you don't have a backup at all. If the data is important to you, then have it stored in multiple locations to prevent data loss.
