The most important part of a pack is fit, fit, fit. Pack fit is almost as individual as shoe fit. The pack needs to fit your GF, it needs to fit the gear she carries and it needs to be comfortable for her with said gear packed inside. I would hesitate to get someone else a pack for this reason, unless rather than the physical pack it's a promise to go with her to get one (and pay for it, of course, lol).
Since you are evidently unfamiliar with fitting a pack, I suggest you read this article: http://www.backpacking.net/gearinf1.html It will give you lots of tips on pack fitting. Note that the most important component of pack size is the torso measurement (often unrelated to height or weight), and this article tells you how to do it. REI has some videos on its website which are good, too.
It's a good idea to start with an outdoor store that specializes in backpacking gear and request a clerk experienced in pack fitting. Consider calling ahead and making an appointment. Even supposedly good outdoor stores don't always have experienced clerks (my experience at REI was downright laughable, but that's another story). Reading up ahead of time will help you judge if the clerk you get knows his/her job or if you need to ask for another clerk. Also, remember that we don't normally backpack with sandbags or weights, which feel quite different in the pack than does gear. Box up all the gear your GF will be carrying to take with you to the store. Don't necessarily buy the first pack that feels good. She should hike around the store (especially up and down stairs) with the loaded pack for at least an hour or two. The clerk should help her learn how to adjust the pack and how to load it properly.
When you find THE pack (from the store or by mail order), as soon as you get it home, load up the pack and hike around the house or (if the weather is dry and you can keep it clean) with the fully loaded pack for 2-3 hours. A very boring "hike," but far better than finding out several miles down the trail that the pack won't work for her and then not being able to return it.
If you order through the internet, be prepared to pay return postage. It may take multiple orders to get the best pack for your GF. (We small women are harder to fit, and she's smaller than I am.) When I went through the process (almost 7 years ago), I lucked out in that the first pack I got fit as though it were custom made just for me. I doubt that my experience will apply to very many people. (The pack has been discontinued, so no point in referring it to you.)
I agree that the pack should have some sort of frame to transfer nearly all the weight to the hip belt. I personally have sensitive shoulders (runs in my family, evidently) and cannot use a pack without load lifters (which also transfer more weight to the hip belt). We women are, after all, built to carry the most weight (think unborn baby) with our hips, not our shoulders! Our center of gravity is lower than that of most men, so the center of gravity of our packs needs to be lower, too.
Best wishes in finding your GF the pack that suits her!