I think I agree with CC, even though the excuses are probably BS by the fauxtographer, as per your statement BEG.
The only thing they can do to have any chance of any refund would be to present any kind of documentation between the faux and the couple. There are so many wedding agreement templates online that you can download for free, it seems silly not to use one.
I could relate to the sister bowing out, understandable, but maybe she should have used that opportunity to hone her skills. Too late now, but just a suggestion.
Sad thing is that the faux probably still has the photos on their drive and their cards. Formatting does not erase the images, it sets the attributes to allow for overwriting, unless you are using software to over-write while formatting (most people do not, since it takes so long). And BEG, yes data can be retrieved from a so called corrupted drive, be it hard drive or solid state. If the hard drive is not solid state and the disks are still operational, software recovery apps can probably recover most of the data, given that it actually corrupted.
I love the “My drives are corrupted, I can’t give you any of the photos” excuse, just shows that they are not a well-managed faux.
Good Luck to them!