It sounds like you have made up your mind already on this, by are holding off because of "friendship". I would just say that the mother's friendship may have been motivated by the desire to keep her daughter in the band--especially since she pulled the "keep her, even if you have to lie to her" routine--

Attitude and commitment matter much more than talent, and most bands hit a glass ceiling because not everyone has the same attitude or commitment. Though you may not see it yet, this person and her mother are tearing your group apart. Things will seem much better the day after they leave--