Hi, thank you for your question. Given that the mother is drinking alcohol and taking diazepam, she should not be breastfeeding at all as this will impact the child health and development. This can be raised as a child protection concern and social services will have a duty to investigate this. This is covered under the Children Act and the decision making of the family courts will consider what is in the child's best interests.
What is your relationship to the mother/family?
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This is classed as neglect and if proved could be sufficient for the child to be removed into care.
It is however unlikely to amount to the Criminal Offence of Child Abuse.
You can read more here
With regard to the GP if he or she is aware of the drinking and the breast feeding then it could amount to negligence to allow the mother to Breastfeed
Please ask if you need further details