Few suggestions - assuming that you really want her as your singer....
Monitors - in-ear would help her to get instant feedback...
She is trying too hard....
Try a different song - I have some songs that are a complete mental block - fine in the car - $hite at rehearsals!
If all that fails, get a new singer! One of my mates is in a band that had a singer/keyboard player - they eventually realised after a lot of audience feedback that she had to go.... Just could not sing!