Her back pain is most likely from poor positioning while seated. Make sure the back of the seat is in the upright position. If it is reclined it will put more stress on the joints in the lower back. When she is seated with the seat back in the upright position take note of the position of her knees compared to her hips. Her knees should be higher than her hips (her thighs will be slanted and she should not feel pressure of the front of the seat where the back of the knee contacts). Use of a small foot rest will most likely be needed to elevate her knees above her hips. Lumbar support will be of minimal help if she does not have proper seating ergonomics.