In addition to all above, tell her it doesn't have to be true! If she's describing her favourite place, or Christmas the examiners won't know if she doesn't really have a naughty little brother or dog who she has brilliantly described!
Checklist. They are looking for accurate punctuation, paragraphing, a variety of sentence lengths and structures, good vocabulary, similes and metaphors . .
Handy hint. Start with a one word sentence. 'Rain! As I pulled back the slightly musty curtains, I watched the raindrops slide down the outside of my window. How could there still be enough water left in heaven for it still to be raining? . . .
Dog hair! When I remember Alfie, the first thing I remember is not his fabulous tail, sweeping everything off the table, nor his chocolate eyes gazing up hopefully at mealtimes, but dog hair! It got everywhere!
Handy hint 2. If you are stuck for an ending, go back to that original word.
Dog hair. What wouldn't I give to spend one more day, cleaning my Alfie's endless dog hair!
HTH