If your friend is using a semi-permanent hair colour, then it does fade eventually. How quickly this fades can depend on the condition of your hair to start with. For instance, hair that is not in tip-top condition tends to be more porous as the scales of your hair have been 'opened' as a result of damage, so the hair takes colour better and may 'hold' it for longer. Also, if your friend is using a light colour anyway, such as a honey blonde shade, these do not last as long as darker colours (I think it is because the colour molecules used in light hair dyes are smaller than darker colours, so they wash out quicker). If your friend wants a permanent colour change, she may need to bleach her hair first slightly (although she is naturally blonde, this still needs to be done to 'strip' the natural colour) then apply the colour of her choice on top. This is best done by a professional though!