I am having the same problem with my mother in law. She has a four inch goldfish in a 20 gallon. She NEVER changes the water and is WAITING for it to die. I have been changing the water every now and then, but I feel like it should be her responsibility to take care of it. All I am doing is condoning her behavior by doing it. Then again I hate seeing it suffer, so I clean the tank. I wish I had an answer for you, and I'm sorry I don't. I do feel your pain though.
Pet stores do take in people's fish but you have to give a donation, because they have to QT them. My mother in law did not want to pay the $5.00 donation they ask for....she is selfish with money.