Have you spoken to BALPA, the UK pilot's union ?
I don't know if you are, or can be, a member, but it might be worth having a chat with them. I would hope that they can advise you or direct you to a solicitor who can.
Good luck, (and come on L3, it sounds as if you have dropped the ball here with this student. You need to get your act together, and look after your students better. You certainly should not be terminating any whom you failed to book into the exams at the right time.)