Does your sister squeeze out the excess liquid from the grated zucchini? This will keep it from shrinking in the center. I just made 4 mini loaf tonight. If you don't mind the extra calories try a crumb topping. I have found it is best to add it after the bread has baked for 15 minutes. This way it doesn't sink into the batter.
1/2 cup regular oats, 1/2 cup brown sugar, 1/4 cup flour, 1/4 t cinnamon, and 1/4 cup butter. Mix all ingredients and crumble on top of loaves 15 minutes into baking.
I am going to try your recipe tomorrow with the cranberries! It sounds great.