Not sure what you mean by an NHS exemption certificate.DO you mean a free prescriptions certificate? If so the answer is no. But if you mean in conjunction with anoher benefit like family tax credit then yes.
if she has to pay the maximum for any course of treatment is £380.
Is she in pain?
Why not go to a dentist who will sedate her with an injection in the arm?
Often it is the case that really nervous people need far less treatment than they think.
Also I have rarely if ever met a nervous patient(assuming a certain level of intelligence) who was not at least partly cured after finding out how painless most treatment is nowadays.Most are completely cured and recommend all their nervous pals to the practice