How can my daughter strip her dyed dark brown hair to light brown?

Use ColorFix, Color Zap, Color Oops, or Igora Phantom. Here are some sites with instructions for using ColorFix and Color Oops. The others are similar. You may need more than one application. After removing the artificial color, you have to re-dye the hair as the hair will not be pretty unless you do.

Hair Color, Hair Care and The No More Bad Hair Days Kit by Robert Craig Salon Products, Ltd.

Developlus - Color Oops