Hi Erin, my daughter has been a type 1 diabetic since she was 2 (now 23) and we were told at the time not to bother with special diabetic foods or sweets as they were basically a waste of time and full of other things that can cause other problems (the runs for example) and if we were going to give her a sweetie or a treat to just do so, and just make sure she ate a healthy meal beforehand, and the treats were given afterwards. However, there is a fairly good selection of diabetic cookbooks avaliable you should get in touch with Diabetes UK as they have a good range but to be honest with you I seldom ever used mine (except to make pineapple lollies). I should imagine that the nutritionist will advise a good healthy balanced diet, which is the one we should all be following I suppose. If your gran is overweight the nutritionist will tell her that she ought to lose weight and advise the best way to do this. Heck it is her birthday, and as long as she doesn't do anything daft and eat the whole cake let her have a normal birthday cake - my daughter does. Your best starting point for advice is Diabetes Uk, and the medical team looking after her at the diabetic clinic.
The people who look after my daughter at Edinburgh's Royal Infirmary are great I hope this is also the case for your gran