Deductions from pay are unlawful unless the employee's contract states that such deductions may be made, or unless the employee has signed a document agreeing to such deductions (prior to any relevant incident occurring). That also applies to 'sending the bill' to an employee, which is effectively a deduction from pay:
http://www.direct.gov.uk/en/Employment/Employe es/Pay/DG_10027228
If there's no existing written agreement, your boss can't make your colleague pay for the damage. It's also unlikely that he could now insist that your colleague signs a document relating to future damage, as that might constitute a change to the existing contract. The only exception might be through a 'disciplinary' route. For example, your boss might say "I'm sacking you because of your negligence through failing to take care of company property. However, I'm prepared to re-employ you, on a new contract, which states that you'll pay towards the costs of repairing any future damage". (That path would be easier to take if previous damage to the company vehicle had already been dealt with by formal disciplinary procedures).
Chris