Need to decrypt files that were encrypted with FileVault

Hi all,

I had a customer bring me a MacBook Pro (older model) running Mountain Lion. The laptop would not boot, and after some trail and error it came to be the hard drive was bad. I was able to get the hard drive to spin up in a USB Enclosure and I was able to locate the user's files (Docs, Pictures, etc...) and put them onto my MacBook running Sierra. During the transfer, the HD crashed and no longer spun up. I'm unable to get any more data off of it.

The files that did transfer seem to be encrypted using FileVault and I am unable to open them on my MacBook. I have since put a new HD in the customer's laptop and rebuild and upgrade to Sierra. The problem is the data that was kept prior to the drive failing completely is not readable by either laptop. I have rebuilt their laptop using the same Apple ID that was used prior to the crash.

So my question is... the few (but precious) files that I was able to transfer to my system before their drive died totally; how can I "decrypt" them so I can put them on their rebuilt system for usage?