Just to confirm, does the user have any access to the data (can they open files in the share)? I would assume not, or decrypting shouldn't be an issue.
In the Symantec Encryption Desktop (SED) GUI, when the user selects the folder, what is shown in the access list? Is it the same as other folders that they do have access to? It may simply be that they used a different key for those shares, which would have a different passphrase.
If it only shows "Unknown Key" in the access list, then the key used for encryption was deleted or removed from the system. In this case there may be no way to recover the data unless they have a backup of the key. I wouldn't expect this to be the case in this instance, since it is saying the passphrase is invalid. That usually means you do have the key, but have forgotten the passphrase.
In a standalone environment, it is important to remember that the passphrase for the key is independent of anything else, so if it was set originally to the same passphrase as the user's Windows account, if they have changed the Windows account password, the passphrase for the key would not change. This is a common misconception, as the keys are not tied to a Windows account. Have the user try any old passwords they remember.