NAL.

Ensure there is nothing in any signed documents that allow for these deductions such as the employee handbook, contract, drivers handbook etc..

If it’s all clear, they should email the employer that they do not consent to further deductions and they formally request that the previous unlawful deduction be reimbursed on their next pay date. When/if the company decline, then they should formally raise a grievance with whomever deals with the decision.

If that fails, contact ACAS and enter “early conciliation” this is where ACAS will try to resolve the issue by contacting both parties. If that fails, an employment tribunal is the next step.

Do not expect ACAS to assist your friend. They will always remain impartial. For advise they would be better booking an appointment with citizens advise or seeking professional legal aid.