The question of the day: Is she really 100 pounds?
We do tend to...umm...stretch the truth a bit in that department....
Anyway, height figures in a little bit, but weight matters much more, along with riding style, rider preference, athletic ability, etc.
If she has been riding 8 days and is just now linking turns, err on the side of shorter. She'll be happier in the end and can always get a longer, stiffer board in a season or two if she decides she feels most at home ripping down double blacks.