Depending on the dementia your grandmother has (likely Alzheimer's), the effects should be reduced with healthy diet and exercise.
Uh...I think you can do a lot, lot better than this. Two interventions that have been studied extensively come to mind. First, treatment with melatonin is very beneficial, provided the dementia is not too far along. Google it yourself, sorry, I don't have references to help you.
Second, a bioactive form of Turmeric called Longvida, which was developed in 2012, has been the target of many studies and is showing a great deal of promise in treating dementia. Here are a few resources to start you off: