Pack straps put pressure on nerves that ennervate the hands and fingers. You can actually develop areas of numbness on the shoulders that persist even after you stop wearing a pack. The pack could also be putting compression on the neck that would impinge on the nerves that run into the hands. Or it could be stretching the nerves that come off her neck by making her shoulders drop further than normal. If the problem occurs even without a pack on, however, it could be something different.

If it is pack related then she needs to change how she loads her pack and/or get a different pack. If she packs the heavier weight too far from her center of gravity, that can change her biomechanics and cause her to stand or walk with an unnatural posture. It might help to take a photo of her standing "normally" (from the front and from the side) and then take a photo loaded down with a pack and see how much her posture changes.

MNS