I believe that paper based medical records need only to be kept for 20 years after the last entry now or for children’s records until the 26th birthday of the child. I have a vague memory that the 20 years is fairly new and that it used to be 10 years after the last entry, so there may be problems gathering evidence, and of course there will be the issue that the people involved will be retired or dead and the culpable organisations may no longer exist.
Can I suggest that a good starting point would be a fixed fee interview with a solicitor? Before this, the person should gather as much factual information as they can, eg names of organisations (and what happened to the organisation during the various reorganisations within the NHS) any names of staff involved, memories of parents or family members and so on. The more base work your friend can do, the better the advice will be from the solicitor and it will save time (and therefore money) if the case goes forward. The GP is unlikely to have knowledge or interest. If they are approached about a complaint, they will pass the complaint on to the practice manager. If its thought that the surgery may have notes of the issue then they can be requested via the surgery’s process. If the Gp is not the GP that the person had as a child, or its not the same surgery then there is no point starting there.