@BoredZelda
And they dont HAVE to let the DD go through that pushes you into your unarranged overdraft or let you draw out money that pushes you into it either, but why would they stop it when they can hammer you with a £30 fee and charge you daily fees.
Of course they do. A DD is an instruction to your bank to pay an amount. If your account has an overdraft facility they can dip in to it. If you don’t want that, and would prefer payments to be rejected, ask for your account to have no overdraft facility. But if you know a DD is going to put you in to overdraft, call your bank.
You've overlooked that she did not have an overdraft facility. Bank accounts
should work the way you assume, but the way most UK banks have always worked is that they let you go overdrawn anyway and then charge you even more than they would if you had wanted an overdraft in the first place.
She would have been better off if the bank had just not made the payment. By making the payment on her behalf, the bank substitutes a debt to itself for an amount owed to someone else, charging her an enormous fee for the privilege. (This is so outrageous that, as a number of posters have pointed out, it has been banned. OP has not addressed how come this is happening when it's no longer allowed.)
It is outrageous that Banks allow payments to create unauthorised overdrafts in any circumstance than a person having explicitly opted in to them doing so. One can get bank accounts that don't, but they are exceptional, possibly because they aren't very profitable for banks.
I also disagree that she should have an overdraft. The best solution to the problem of outgoings exceeding income is to stop the outgoings, make the original debtor wait for the money, not switch the debt to someone who will charge you more for the privilege.