I agree... this was story with a great outcome. Nobody should feel guilty, as your sister did as much as she could do in her power! If she feels bad, then she might be able to try and locate the people who generously took him to the vets. Yes, their methods were a bit brusque, but I think what they did was commendable! Perhaps your sister can make a donation in Tux's name and she can explain (or not) what had happened. I sure hope she "shared" the letter with her clueless friend.

And be happy that you are a new Aunt!