As far as what to do, I don't know what to say about this. How horrible for your sister. I'm so sorry this happened to her. If she feels that her cat would still be alive if the vet had kept it at the office, then you probably should report this incident to the state licensing board for follow up and documentation purposes.
Perhaps one of "our BB vets" will weigh in on this issue and have some idea of what action, if any, is appropriate.
Again, I am very sorry for your sister's loss and the cat's suffering.
To take the first step in faith, you don't have to see the whole staircase: just take the first step. - Dr. Martin Luther King