I made my dress. It was 2 pieces: A circle skirt and a corset/bustier type top. I used the roethke tute for the top and just drew up a few extra angles for the boobs.
Unless your friend is heart set on a certain type of dress, you may do better with two pieces. you can mix and match styles, giving you a huge amount of options. Also, check out burda styles for cheap or free patterns. They have a pretty decent plus section, and even one plus formal dress. I have quite a collection of their patterns, so feel free to PM me for more info.