This normally requires the user's PGP passphrase that the disk was encrypted to. However, if you are in a PGP Universal managed setting, you have have configured PGP Universal to generate a Whole Disk Recovery Token, that could be used for access. There is a possiblity that the user may have given someone else passphrase access, and there is also a chance that the user used a week passphrase that you might be able to guess.