How terrible and sad this story is my thoughts are with your mum xx
I don't understand why the groomer called and didn't explain the poor dog had passed. They already sound like they are covering themselves. That dog should never have been left alone chained up on the table.
Definately great advice above but I would also have some legal advice as well incase the grooming company plays hard ball. Sounds like a terrible accident but also it could of been prevented. At least speaking to someone in the legal field can tell you where you stand and what you are able to go ahead with.
Xx