Don't tell her to slow down. With all due respect, you don't know anything about stuttering and we've all had unsolicited advice like that and it's helped 0 of us. Saying slow down only adds to the stigma and reinforces that her stuttering is not ok, which further adds to frustration. You can talk with her about her stutter or ask if it's ok to ask questions/talk about it and be supportive. She can see a speech therapist who understands stuttering but finding one that can help more than hurt is the challenge. I would avoid those who will work on fluency with her