It depends on when the unauthorised purchases had taken place.
If the individual was still employed at the time, then it would be reported on the P11D as a benefit and for national insurance through the payroll. You can find guidance on this here:
Expenses and benefits: credit, debit and charge cards
If the individual was not employed at the time they have used the credit card, then you will need to resolve this between yourselves.
If you cannot get the money returned from the individual, then you will need to write in to HMRC to advise that a benefit has been provided to someone who is not an employee of your company.
If you need further advice on this matter, you may wish to contact the Employer Helpline on 0300 200 3200.
We can then clarify the exact position with you over the phone.