Did you know dementia affects the taste buds? Apparently it makes most things taste very bland, and the sweet taste buds are some of the last to go, so developing a sweet tooth is very common. This could explain why your nan has lost her taste for savoury foods. I've seen some diabetic-friendly menus and I was surprised at the amount of puddings etc deemed acceptable - but apparently it's more about the combinations so a pudding is ok as part of a meal or custard or ice cream ok with cake as the fat and protein slow down the release of the sugar, etc. Eating hard boiled sweets may not be so ok, but biscuits again may be ok especially with some fat like a bourbon or something like that. If staff are testing her regularly they can ensure she has enough medication, too. But unfortunately both dementia and diabetes are chronic, so they can't be fixed and as Martin said at this stage of life it's more about enjoyment than long term health.