What an awful situation!
I would recommend two things (no idea if either of these will work, just what I would try):
First, maybe your sister can go back to the pet store and offer some nominal amount, maybe fifty dollars. Have her ask what happens to animals who are never purchased. She can give them her name & number and ask them to consider her offer before they dump the animal. I would do this as nicely as possible. The point is not to condemn the pet store, but to help the dog.
If that doesn't work, and you have time, seriously, I would picket. Even a couple people standing outside and handing out flyers is very embarassing to the store. At the very least, they will probably move him to a bigger cage to get ride of you.
It's so tough to see animals in pet stores.
