Start small and work up.
Give her small assignments and mark them - with lots of praise. Wroking up to the big final assignment.
When she feels comfortable wih you and knows you aren't judging HER just her writing she will do alot better.
I also find that stickers are a great motivator - even for my year 13 students. They complained when I forgot to give them a sticker for getting higher than achieved level. Hee hee. (Mind you they WERE sparkly Whinnie the Pooh ones...)