how do you motivate someone with stage fright?

[Staccato]

There's several good answers that may fit. Practice performing in front of a small gathering in a practice setting. Do this multiple times, and take it public to a small crowd. It will get easier, especially if she performs by focusing to a few familiar faces in the crowd-like friends, or family (or one person). She can practice performing to them, or while looking at the ceiling (or looking at the back wall of the place). Ultimately, the attitude is to smile, and enjoy what you're doing.