The Post Office card account is only used for benefit payments.
Unless you need to keep going into the branch, Co-op is best. You could try opening the account over the phone. They allow you to use post offices to pay in cheques etc. and to draw out cash, and normally give you an Electron card.
I don't understand what you say about NatWest. If the account is in his sole name (i.e. it is not a joint account) and the overdraft was incurred before he was bankrupt then it is included in the bankruptcy and he is not allowed to make any payments towards it to NatWest. He would be breaking bankruptcy law by doing so.
However, if the account was not overdrawn when he went bankrupt then the overdraft is not included in the bankruptcy and will have to be paid off at some time - probably by small monthly instalments if that is all he can afford.