Hair and eyebrow colour are different?

I am asking this question on the behalf of my friend, he has a sort of chestnut brown colour hair and at the age of 11-12 years old his eyebrow colour had matched. In recent years I had begun to notice that his eyebrows were getting darker and darker and now they're completely black as opposed to his brightish natural hair colour. Does anyone know how this could have happened and if possible is there any solution?