1. The bank will not close an account which is overdrawn.
2. If nothing is done to notify the bank of the problem, they will start recovery action at some stage - first of all by writing letters. If they get no reply they will either pass the debt to debt collectors (who will then start writing more letters) or maybe start County Court action. If they do, it would be inevitable that a County Court Judgement (CCJ) would be made because your daughter & husband cannot deny that they owe the money.
3. If the terms of the CCJ are broken (eg if it says your daughter is to pay £x per month & they fail to do so on time) the bank can start enforcement action, which can include sending bailiffs. Up to this stage, no bailiffs can come - although it is not unkown for debt collectors to send people who try to pass themselves off as bailiffs. Such people have no rights of any kind & should be sent on their way.
4. The best way by far to deal with this is to let the bank know now what the problem is, & to ask them to accept a nominal payment (such as £1 per month) and freeze interest and charges until finances improve.
5. If they need help with this they should go to the local CAB or ring CCCS. Do not get involved with any firms that make a charge for their debt management "services".
6. They must stop using the account (if they haven't already done so).