As an anaesthetist and pain specialist doctor. Whilst this may not have been the best course of action on your GPs part (with the benefit of hindsight) it was not a course of action that would be uncommon. The alternative medications are generally more addictive than codeine and would expose her to the same risk. The liver failure part of her overdose was almost certainly down to the paracetamol component anyway. Had she left the doctors with no treatment whatsoever due to her previous addiction the doctor would be labelled as stereotyping and judgmental.

If your mother has capacity to make decisions for herself then her taking of the more that prescribed dose of medication was on her, not her physician, his prescription and recommendation was perfectly reasonable.

Whilst I understand and empathise that this is a tough situation to be in and your mother has clearly been through a difficult situation, I think it is unlikely that you’ll find a case for the doctor being negligent... because they weren’t.