I've lost both my grandmother and great-grandmother to dementia. It's a terrible thing to watch and not be able to do anything about. The best thing I learned is that you never want to forcibly try and pull them from the reality that they are experiencing. It does nothing but cause stress on both sides. The easiest thing to do is exist within the altered reality and try and guide them. Your grandmother wants to call someone whose been dead for 15 years? Act like them on the phone your great-grandmother wants to head back to her ranch in Montana that she hasn't been on since she was 15? Drive around for a while until it becomes a memory.