Decrypt an external hardrive FileVault
There is no need to decrypt the FileVault password with a 3rd-party tool like Passware Kit Forensic. You just have to unlock it with the hopefully known password/passphrase.

You may either:


Plug the external case containing the hard drive into another Mac and try to boot from it. Then revert the encryption in System Preferences > Security.
Plug the external case containing the hard drive into another Mac and try to revert the encryption by entering the following Terminal commands:

diskutil corestorage list
diskutil cs unlockVolume lvUUID #UUID of the encrypted logical volume
diskutil cs revert lvUUID #UUID of the encrypted logical volume


Consider that the Mac may also have a CoreStorage Volume Group.


Requirements:

The Mac has to be bootable with the "FileVaulted" system (i.e you can't use a PowerBook or an old iMac).

If you want to further process the data of the now decrypted drive with a PC, you have to install a software/driver in Windows capable of at least reading HFS+ volumes.