It is an offence to get credit of more than �500 while bankrupt without letting the potential creditor know about the bankruptcy so, if this is what happened, it is clearly the bankrupt's fault.
Was the first lot of credit given before the debtor went bankrupt? If so, that debt is included in the bankruptcy & the creditor would have been informed by the Official Receiver that the debtor was bankrupt (assuming the debt was listed on the bankruptcy form - if it wasn't then the bankrupt is at fault).
On getting that information, the creditor should have closed the account. If this was not done and the debtor went on using the store card then:
a) The debtor can be at fault - see first para. above; &
b) The store has made an admin. error, but that does not mean the debtor can avoid paying the debt back. This is because any debt incurred after the date of the bankruptcy is excluded from it and still has to be paid.