Think of the original pricing. If someone is stretching their budget to buy a 2012 Model S used, I'm going to tell them they are an idiot same as I would tell someone stretching to buy an S-Class, Range Rover, or a 7-series, or really any car that was $100k new and $20k at eight years old. Things will probably go totally fine, but they might be buying a piece of driveway art.
Back to the OP, I think Tesla should step up and goodwill this. It's a car barely out of warranty that has already had a complete battery failure. At least do a 50% split or similar.