As humans, and even more so as kids, we need external validation. Lot of people that stutter end up having psychological trauma because they don’t get this external validation from the society, and carry this trauma all their lives. So, aside from all the great advice from all the comments, also look into teaching her self-validation. For example, Her opinion on herself has more weight compared to what her friends might say. A good therapist should be able to help you both on how to put this in practice