How to make dirty blonde hair brunette natuarlly?

My cousin has dirty blonde hair and wants to be brunette super bad. Her parents don't want her to dye her hair because of the dye/damage/cost. So is there a way that she could turn in medium brown (more on the dark side of medium brown) natuarlly? She is staying with me for a week, we have thay much time. I've heard coffe works?