One of my Thanksgiving guests can't eat dairy, wheat, oats or corn. Any dessert suggestions?

Quince should be in top form at Thanksgiving. You could make a quince sorbet or this recipe for quince in syrup http://www.epicurious.com/recipes/food/views/Quince-in-Syrup-15416

When you cook quince, they turn a rosy hue which looks so pretty.

I second the poached pears, chocolate mousse, and the meringue. You could also do some fall flavored sorbets.

This Sweet Potato Pudding is super simple and fits the Thanksgiving theme. I made it yesterday and my family devoured it. If you want to skip the coconut milk, you could substitute soaked cashews as long as you have a Vitamix (or similar) to create a silky texture.
http://www.choosingraw.com/five-ingredient-maple-sweet-potato-pudding/

This one isn't very "Thanksgiving-y" but it tastes amazing! If you don't want to use the fruits, the 'crust' is fantastic rolled into little balls like small truffles. http://blissfulbasil.com/2014/03/03/raw-blueberry-and-kiwi-tart/

Also something like this is a great idea -- you could even sub the strawberries out for pumkin if you wanted to experiment: https://food52.com/recipes/22884-raw-vegan-strawberry-vanilla-cheesecake

Make an almond flour pie crust and use fresh frute and chocolate AS a filling

It's not very thanksgiving-y. But lemon-curd with fresh berries is so yummy and can be made in advance.

Make a pavlova (sugar & egg whites) and top with non-dairy whipped topping (like sweetened whipped full fat coconut milk, like this one: http://www.glutenfreegigi.com/dairy-free-coconut-milk-whipped-topping/)

and fresh berries.

Some cyanide! I would kill myself if I couldnt eat these things!

You could make the Calabrian Walnut Cake Emiko posted today: https://food52.com/recipes/31722-calabrian-walnut-cake-torta-di-noci (Wholesome Sweeteners makes a corn starch free powdered sugar). Another option would be to take Gena Hamshaw's vegan pumpkin pie and cook it as a shell-less custard (or with a gluten-free crust) instead: https://food52.com/recipes/19560-you-won-t-believe-it-s-vegan-pumpkin-pie A third option would be a fruit crisp using almond flour, shredded coconut, chopped nuts, and coconut oil to make the topping -- I'm a fan of combining frozen raspberries, fresh pears, candied ginger, and cardamom or cranberries, apples, and Chinese 5-spice.

These are excellent suggestions. Thank you all for your input!

I love the Herve This chocolate mousse recipe right here on Food52: https://food52.com/recipes/16044-herve-this-chocolate-mousse (Whipped cream, coconut or not, optional :))

I am loving the poached pear ideas, and they are so seasonal! You could also do a rice pudding with almond milk and apple.

This chocolate tofu pudding from Mark Bittman:
http://www.nytimes.com/2009/05/20/dining/201mrex.html
He has said it's one of his best "Minimalist" recipes, and it doesn't taste like tofu at all. It's written with cinnamon and chili for a Mexican chocolate flavor, but I imagine vanilla or orange liqueur would be a nice alternative.
It's also so easy that you could serve it with the poached pears that several others have recommended.

Extending Drbabs and Queen - Slow poach pears in red wine with spices, pull the pears when they are cooked but still firm. Strain and reduce the syrup. Serve this with dairy for those who want it (whipped or ice cream). Or serve it as part of a dessert assortment - figs, candied citrus, nuts (some plain, some spiced) and winter cheeses (stilton, etc.). Offer a lovely citrus-note tea and a port - - I'm ready to skip dinner and go straight to dessert!

Poached pears (in either red or white wine, with a wine reduction sauce or chocolate sauce) or baked apples ( we love 'em stuffed with toasted walnuts and caramel) came immediately to mind.

chocolate avocado mousse (foodies.ca) or Mousse au chocolat et a l’huile d’olive (see negevnectars.com) and serve garnishes to taste for those who can eat them...tuiles, nuts, whipped cream, dried or fresh fruit.

Susan's answer is great, but I am so not a fan of coconut. I'd make baked apples or pears with cranberries maybe and a nut topping.

How about tiny marzipans, or chocolate and caramel covered dried fruits... Even better, you can stuff the fruits with marzipan and then glaze with caramel or bittersweet chocolate. Works great with dates, figs, apricots...

Pumpkin or chocolate mousse comes to mind. If you google paleo mousses, you will find recipes that use canned 100% canned coconut milk in place of dairy. It can even be whipped as a topping. Nom Nom Paleo and Melissa Joulwan (aka Clothes Make The Girl) are two very reliable sources. I think Nom Nom has a pot de creme recipe.