what about that great magical one-ingredient icecream? (blended frozen bananas) No sugar additives there, but of course there is the fructose in the bananas...
You could find a good sugar-free pie base recipe and make an 'ice-cream' pie with it :)

What about these mini pumpkin phyllo tarts? They aren't sugar free, but they are low carb (6 carbs per tart). Your grandfather can eat a couple, but they are also perfectly festive and sure to be a hit with everyone!
http://tutti-dolci.com/2011/10/31/pumpkin-phyllo-tarts/

Similar to the older article link, I'd say just look up Splenda based recipes (Splenda won't mess with insulin levels) It doesn't sub out 1:1 for sugar ,especially in baked goods, but it is a pretty good substitute.

You can make pumpkin pie with Splenda instead of sugar. The volume is less, but the taste and texture are good. Anything which is egg-based, cream or milk-based adjusts well to sugar-substitutes. Custard, pudding, and cheesecake are fine with Splenda instead of sugar.

I've got a collection of sugar-free recipes here:
http://sweetenersandlight.blogspot.com/

I've baked/roasted plain sweet apples with just a sprinkling of cinnamon and they were lovely, even without the sugar. With a little unsweetened whipped cream or sugar-free vanilla ice cream, they could be yummy. Like pie, but without sugar.

Dried fruit also makes it's own kind of syrup if you soak it overnight. Not sure what natural sugars in fruit do to blood sugar levels, but they are definitely sweet enough to eat without added sugar.

Clementine oranges with a few slivers of very dark (and not so sweet) chocolate and some plain roasted nuts like pecans or almonds would also be lovely.

You could also make no-bake desserts like rice pudding or custards with sugar replacers fairly easily, I'm sure.

Lots of sugar-free recipes here: http://genaw.com/lowcarb/dessert_recipes.html

The myth that people with diabetes need to eat sugar-free desserts persists, but all carb sources provide sugar. Bananas, bread, potatoes, candy. It's about moderation and management. Having said that, if your grandmother was diagnosed a long time ago, she may still believe she needs to go sugar free (or she's tired of others who don't understand the guidelines policing her dessert choices). So a dessert that uses Splenda may be welcome. But a slice of normal pumpkin pie isn't going to be any harder on her blood sugar than a serving of mashed potatoes or stuffing. FYI, you can find reliable options at dlife.com.

I've made a pretty killer beet and goat cheese tart. I like to put about 1 tbsp sugar in the crust and a honey balsamic glaze on top, but these could be skipped if necessary.

I've had super results lately using Stevia-in-the-Raw or Ideal sweetener made with xylitol. Ideal comes in 'white sugar', 'brown sugar', and 'powdered sugar' varieties, and, and both brands are used 1:1 for easy substitution. I just took some fudge off the stove that I made with sugar-free chocolate chips and home-made sweetened condensed 'milk' made with eggs and Ideal.

I was going to suggest baked apples with just a bit of brown or maple sugar and cinnamon sprinkled on top, served over sugar-free vanilla ice cream.

Try this source:

http://www.lauraleacooks.com/search/label/Desserts

Here are a couple of books to check out from the library:

http://www.amazon.com/preventions-decadent-desserts-diabetics-luscious/dp/1579547001

http://www.amazon.com/Big-Book-Diabetic-Desserts/dp/1580402747/ref=pd_sim_b_1

Recipes from the Prevention diabetic cookbook:

http://recipes.prevention.com/Recipes/SearchResults.aspx?WithSource=Prevention's+Decadent+Desserts+for+Diabetics

No sugar added apple pie seems to go over pretty well when I am in these situations. I just go crazy with spices and make a buttery crust.