I'm 5'4", and I love my Golite Quest...size small is just under 4000. Sunny Sports has one for $100. It's very comfortable, easy to adjust and one of the great features is that it's equipped to be cinched down to use as a daypack (I've done it several times and found it very usefull). It's also very water resistant! The Odyssey is a very similar pack with more storage capacity, and I saw one one steepandcheap for under $100 the other day - they might reappear, or Backcountry might have some for nearly that price. I think they come in around 4700cu in.
Tell your girl to enjoy her dendrochronology trip!