It is not against the law for an undischarged bankrupt to have a bank account without permission from the bankruptcy court, unless the bankrupt is subject to a bankruptcy restriction order which specificallys states so. The problem used to be was that banks would not offer a bank account to those who are undischarged, but now many do. A bankrupt is automatically discharged after a year unless there is a BRO.
On bankruptcy, your debts crystallise at that precise moment. Any assets you owned at the time of bankruptcy are taken into account and used to discharge creditors. Unless the bankrupt is subject to an income payments order, his earnings are his to keep and there is no duty to work to pay off the debts included in the bankruptcy.
The bankruptcy limit is £5k since 2015, but before that it was only £750.
I would suggest that explorer closes the account. I cannot see there are any offences under the Insolvency Acts but there may be other offences.
Explorer, try not to worry and just write to him to close the account. You could also go to this website
https://www.gov.uk/government/organisations/insolvency-service and check the status of your friend's bankruptcy.