Stop telling her to slow down. It doesn't help and if we wanted to slow down we would. It also reinforces that stuttering is not ok because if it were you wouldn't be telling her to slow down in order to be more fluent. Secondly, if she's open to it, I highly recommend getting her involved with FRIENDS. It's an amazing organization and I can't stress the important of finding community with other people who stutter. It's largely for teens but they also have information for parents. Their annual convention is actually coming up this July and I think she would get more out of it than anything else you can find. She'll make friends, see confident role models, and have a great time. You could make a family trip out of it and Chicago is great in the summer. https://www.friendswhostutter.org/convention/