Any advice for my mother-in-law?

She has an older cat, a persian, who is about 12. He has pee'ed on the carpet constantly for the past several years. My mother-in-law is 82 and wont listen to my advice to get rid of her tiny littlerbox and get one that the cat actually fits in. On a few occasions, I've seen him try to use the box but it's so small that his just hangs over the edge and he totally misses the mark. Now he doesnt even try the box anymore and has been pissing on the carpets for about 6 years. Her house, needless to say, reeks and makes me want to vomit. Her solution is to just keep moving the litterbox around the house, presumably so he wont smell his own pee anymore and will use the box again. It doesnt work. He will follow the box wherever it goes, but makes no attempt to even crawl into it. He just pees next to it. So, naturally, every room in her house reeks. Well, now she has moved into a nice rent house and, although the landlord is aware of the cats, he has no idea that one of them is a chronic carpet-pisser. She neglected to tell him that. Her new house is starting to stink and I know the landlord will figure it out the first time he goes to visit. Does anyone know why a cat would prefer to piss on the carpet, and will even come in from outside to pee, and then go right back outside? Does anyone know how to stop it?