She will have a great time.
She can either arrive wearing it - or, if it's more "magical for her", when she arrives at BBB, they have changing rooms where she can get changed before her appointment. I suppose she could also change after... but they do a nice "reveal" when they take the protective drape off, spin the chair around, and the girl gets to see herself in the mirror for the first time with her new hairstyle. It's fun if they are wearing their costume, and makes for good pictures.
They have photopass photogs there taking pictures during, but you are welcome to take pictures of the transformation too.
But she can do whatever she is most comfortable with.