Taking dynamics away from the uncertainties of note reading may just help your student loosen up enough to play forte (or even fortissimo!).
Ignore your student’s Pianissimo Preoccupation in her pieces and focus on dynamics only in her scales. Better yet, only in scales she already knows very well.
Ask her to play the scale with a crescendo ascending, and diminuendo descending. Encourage her to make this as exaggerated as she can.
Next, reverse the dynamics, asking for a diminuendo ascending and crescendo descending. This will probably be more difficult for her as she will need to play loudly without building up to it.
Assign practice of all her scales/exercises in this way. Wait until she is doing this confidently in her scales before asking for these dynamics in her pieces. This may take several weeks or even a few months of encouragement but stick at it.
Pianissimo Preoccupation: Roar!
Clearing her throat (as it were) before playing will help your student to play with a little more conviction as in contrast to the Roar! mezzo piano will sound almost as quiet as pianissimo did before it.
Ask your student to play the very first note, notes or chord of a piece she is working on – as loudly as possible. Describe it as a lion’s roar, that’s how loud it should be.
After this “throat clearing” she can start her piece as normal.
Do this for every scale, exercise and piece in the lesson and tell her to do Roars! before each and every assignment too.
Continue to request a Roar! before each piece at subsequent lessons until the Pianissimo Preoccupation improves.