Some lifters have trouble hitting their backs, even with back exercises. Often, the biceps take over. The unilateral lat pulldown allows you to really focus on the back one side at a time.
Do not fully extend the arm at the top of the movement.
She did 15 minutes on the elliptical trainer. Then she did arm curls, leg extensions, straight leg raises, and side bends-lots of side bends. In short, she was doing everything wrong.