Considering the age of your daughter, the possibility to have black hair again is good. She must take care of your diet first, stop junk food completely, along with sugary beverages, colas, soda, excess tea/coffee. Include Antioxidant-rich foods : Fresh fruits and vegetables, whole grains like oats, brown rice, barley, whole wheat, amla, green tea, olive oil, fish, flaxseeds etc. For Vitamin B12- include seafood, eggs, and meats are good sources of it. Massage her hair with coconut/ olive oil with curry leaves added to it. Ask her to Practise yoga and pranayama daily.