If the files were encrypted, the only way to recover them is to recover your friend's security key and install it on your computer so that you can decrypt the files.
If it is simply a matter of ownership/permissions, you can take control of the files if you are an administrator on your computer. To do this, follow these steps:
Right-click on her drive in My Computer and select Properties.
In the Properties window, click on the Security tab then click the Advanced button.
In the Advanced Security window, click on the Owner tab.
In the list of users below "Change owner to:" click on the user name that you are logged in with on your PC and check the box labeled "Replace owner on subcontainers and objects" and then click OK.
This will give you ownership of all non-encrypted files on the drive (you still need her original security key to access any encrypted files) and you should be able to back them up normally.