Dementia are diseases that causes memory loss. It affects individuals daily life very huge. One way to make them understand is to show them and explain to them daily. This may need a huge patience for you but this is the only way.
Well as a vegetarian i would say broccoli and a lot of greens like kale, spinach, green beans. fruits are a must as well. i like to eat oranges and strawberries. i make a smoothie everyday for my morning run. anything organic is good too.