There are several possible explanations, but the most likely is just down to simple genetics. Your friends mother and father have some genes for blonde hair and some for dark hair. The genes for dark hair are dominant in the mother and father, but they just happened to pass on their blonde genes to your friend. Another possibility is that your friend’s hair will change to being darker as she gets older. Or it could be that she dyes her hair blonde. Or that her family dye their hair dark.