This can be a really tough one. One exercise IS to draw and hold without an arrow and THEN do the same thing with an arrow but she cannot shoot! Tell her she MUST just let down -- she is not to let the arrow go. Then if she does anyway, it's back to no arrow for several repetitions, then try the arrow again.
You don't give too much detail, but her age, bow weight, whether she's shooting at a target or blank bale, etc all can have an affect.
If she is shooting at a target, take it down and shoot blank bale. And use the exercise on a blank bale.
A very general answer to your question. Are there any coaches in your area? That would be your BEST bet.
Arne