That's a bit of a tough order. If she was really serious there would not be dessert, so I take it that you can have a few calories. Citrus is in season so that may be a good choice.
Angelfood is the classic when you need a tsuper low cal dessert and I just love the Tangerine Angelfood by Gale Gand.Often people don't realize how good an angelfood cake is because they haven't had a homemade one. Plate some kind of fruit along with it.
I was very happy with a lemon pudding I recently made from Luscious Lemon Desserts. This is not meant as a low cal recipe, but it does not have any cream or crust where calories hide. You can put a spoon of whipped cream and serve it with a little cookie if you serve it in small cups, but it doesn't really need it. It is just delicious and refreshing, and both light and rich at the same time, if that makes any sense. The original recipe calls for 2 Tbsp of zest but I think that might be too intense. I just used the zest from a large lemon. I made the mistake of not straining it, so that may be the source of the intensity. I would strain it to make it silky smooth. I would also serve it the same day or next at most.
3/4 cup sugar
1/4 cup cornstarch
3 large egg yolks
zest from one or two lemons
pinch of salt
1/2 cup fresh lemon juice
2 Tbsp unsalted butter, room temp
1) Whisk sugar and cornstarch together in a medium saucepan; add milk and whisk until smooth. Add egg yolks, zest and salt and cook over medium heat, stirring frequently as it heats, then constantly until thickened. Should take about 5 minutes.
2) Remove from heat and stir in lemon juice and butter. Pour through a strainer into a bowl or individual dishes. Let cool and refrigerate 2 hours or up to 2 days.
I think the Pavlova is another really good idea if you can get some really nice imported berries to top it with and if you have the ability to assemble it onsite as it does not keep well assembled. I once had a wonderful chocolate pav topped with whipped cream, blackberries and ki wi. There are some nice sone I saw by googling.
Here's a low cal one from Gale Gand and if you scroll to the bottom there is a recipe for individual angel cake made in muffin tins and stuffed with berries and liquor.
How about an angel food cake trifle? You could make the cake (or buy it if you're pressed for time), cut up into cubes, add fresh mascerated berries and a little whipped cream. Layer in a large glass bowl. Makes a lovely presentation and hopefully fits the bill for a light dessert.
re: chocolate chick
I saw a recipe for a pumpkin trifle that looked great. You layer angel food cake with sweetened/spiced pumpkin purée lightened with lowfat Cool Whip. Then you top with more Cool Whip and some toasted pecans.
You can also make wacky cake by replacing the oil with diet cola. It works really well with chocolate cake.
Last but not least: fruit with a sweet yogurt dip. Make sure you use Greek yogurt (maybe Fage 0%) for extra creaminess.
Composed tropical fruit salad - beautiful presentation can make up for less-indulgent ingredients - we eat with our eyes too! Lay out a row of each fruit on a platter (e.g. pineapple, mango, papaya, etc), with a bowl lightly sweetened crème fraiche (you can stir in some vanilla beans if you want to be fancy) for those who aren't sticking to their resolutions ;-) Serve with a nice liqueur - amaretto or limoncello would be particularly good.