They need a better system for changing and disposing of litter. Cat litter can get very expensive if we’re talking about multiple cats. People don’t like to dump it all out every day because of the expense but they need to be doing that. Maybe offer to donate litter if this is the issue? Or heavy trash bags if it’s the trash that stinks?

They also need to make sure all of the cats are spayed and neutered. Unneutered males stink to high heaven.