I love the idea of the halibut. Would you consider serving with some zucchini ribbons? I always think those can be done delicately and taste wonderful with the halibut.
How about served with a red bell pepper harissa? Is she open to spicy? That would tie the halibut in nicely with the couscous.
Serve it with a nice Riesling.
For dessert, I was thinking something like a fancy fruit with a grown up granita.
http://www.epicurious.com/recipes/food/views/Asian-Pear-and-Grapefruit-Salad-with-em-Sake-em-Granita-and-Pear-Sorbet-354894
Or perhaps madelines with a sweeter granita:
http://www.epicurious.com/recipes/food/views/Pink-Grapefruit-Strawberry-and-Champagne-Granita-with-Sugared-Strawberries-109409
That will be fresh and nice after the fish. Granitas, while time consuming, are not labor intensive and are delicious.
Have fun!