If this is performance anxiety, talking through it or taking a break or any number of performance therapy will help.

If this is anxiety disorder, none of that will help. Even if she stops dancing, her mind will latch onto something else to be anxious about and the process will repeat with a different activity.

It seems worth while to me to get her into a couple of therapy sessions with a licensed professional psychologist who will most likely be able to differentiate and to take appropriate steps. Either way, the therapist can help you understand what is going on and help you to help her.