So lets back up a moment here your Aunt sent the SSD out as she needed to recover the stuff on it. So was the system really working at that point? I find it odd to go to such an expense to gain access to the data when you could have simply plugged in an external drive and copy off everything.
Something else is at play here. FileVault encryption is disk based. You can either protect a folder or the entire drive. It stays with the disk. One of the element of the encryption is the CPU's serial number so if the drive is transferred it can't be used. So if the SSD you got was encrypted then that can explain why if was not working (used).
I don't do full drive encryption and I'm not sure what Apple has done in the newer systems. But from my use for folder encryption it wouldn't have stopped you using a new SSD in the system.
In any case you would have been asked for the recovery key some where in the boot process so if you had a dead system you could access the drive using the special key. Review this: How to encrypt your Mac with FileVault 2, and why you absolutely should.
So where does that leave us??? I'm thinking the system had a problem to start with. Was the logic board damaged at some point? Liquid spill.
At this point I think you'll need to visit an Apple Store and let them see if they can fix things. You'll want to put the SSD back in so they don't get a surprise ;-}
I would be curious what they say and do to fix things. Don't over volunteer to much. Just say it's your aunts system and it stopped working. As they say less is more!