What should I do about this elderly cat?

I posted two unrelated questions with different issues about our animals and how careless the owner is. But, this cat is in one of the worst shapes. She looks as if nothing is wrong with her and she is extremely old. She hardly gets any attention but today I decided to go and spend some time with her. Her fur is EXTREMELY dirty. I cannot express how dirty it is. The owner claims "she is too old to clean herself" but that's really not a good thing. She has very long fur and many sticky, hard substances are getting caught in there. She also cannot enter a litter box. We tried buying one without a lid but the other cats use it and she won't interact with other cats. She won't even eat out of the same bowl. So, she decides to use the bathroom on the floor. It's really bad. Not sure what to do. If you tell me to go to a vet... sorry to say, I'm 12 and the owner isn't the best owner out there. I've discussed this problem but she doesn't do anything about it. She won't even take my dog to the vet to check for parasites even though he had them before and he still had eggs in his stool.