The employer could claim this as a deduction for 'missing stock'. Such a deduction has to be recorded on the wageslip. Your grand-daughter could reasonably ask for proof of the value of the goods.
This sounds like an unauthorised deduction from wages and may happen again and again.
The Employment Rights Act 1996 and Wages Act 1986 should protect a worker who tries to enforce her right to receive in full what she has earned under this legislation. A worker who is dismissed for trying to exercise her rights under this legislation can claim automatic unfair dismissal irrespective of how long she has worked for the employer because she has been dismissed for trying to claim a statutory right.
Contact the ACAS Helpline 08457 47 47 47 for more detailed advice.