Timid suggestion from an amateur who sings a lot...

A couple of times when much younger I had this problem. It was related to performance nerves (and an audience of one is plenty to start this up), and I liken it to 'jumping in' when joining a skipping rope game, where nervousness about missing causes you to freeze up on gauging the right moment to 'jump'. Maybe some relaxation techniques would help her - or s good stiff drink!