That said, it's true that an external-frame pack in general transfers more weight to the hips than an internal-frame pack, although that's a bit of a generalization and susceptible to counter-arguments. Internal-frame packs do tend to hug the back, using the back and shoulders more for load bearing. They're also "softer" and more forgiving, so may have advantages here.
These days, it's my generally accepted view that the best external-frame pack on the market is the Dana Design Longbed ($299). Really, it's a fantastic pack-big (5,600 cubic inches), easy to pack, extremely comfortable, with all the load-carrying advantages of an external-frame pack without the excessive stiffness found in some models.
It is, alas, on the expensive side. For a good pack that costs quite a bit less, take a look at the Kelty Super Tioga SE ($185) or the Jansport Olympic ($150). Both are well-made packs that are more in the tradition of external-frame models. That is, a bit on the stiff side. But they're excellent at transferring the load to the hips.
Otherwise, the big deal for your girlfriend will be to pack as lightly as humanly possible. That can be a bit of a challenge for someone who weighs 100 pounds, as she'll need to carry her share of group supplies plus the full complement of clothing. But trimming ounces will do a lot for her lumbar health.