System Settings are controlled with the Gamepad. The question "Do X without the Gamepad?" comes regularly. I don't get it. This thing is an important piece of the Wii U and it is often lost or defective. In your case the solution is easy: Open the pad and insert the battery (and charge it). Are the screws stripped beyond believe or what? (Please upload picture) They are normally easily unscrewed.
Also: Your sister is very smart! Taking out the battery was the best she could do before putting away the Wii U for a longer period of time. The Gamepad would have deep discharged the battery otherwise and self discharge afterwards would most likely have destroyed it by now.