If the Mac is encrypted with FV2, and you have no Recovery key, and the user doesn't remember the password FileVault is expecting, the only other possibility is if you've set up an Institutional key with your FileVault configuration and the Mac received that. The Institutional key may be able to unlock the Mac's HD. If that isn't an option, you have run out of luck. All data will likely be lost on it, since there is not going to be any other way to get into the drive.
On the other hand, what does the login screen look like? Is it a standard username & password field login, or a user image with a password field beneath it. Its not clear whether you're looking at a FileVault login screen based on what you wrote.
