I am still a little confused, so forgive me for being dim!
Are you saying that the gentleman in question grew up believing a certain lady to be his granny but she was (or perhaps was not!) his real mother? Or did this information come to him later in life?
And is it the case that he has subsequently been told (or discovered) that his real, natural mother was this lady's daughter?
Adoption records (which, as I said before, your friend's father is legally entitled to see, though he will have to go through the formality of a session with an counsellor first) may not be all that helpful. If granny/mother registered him falsely as her own, to protect her daughter's reputation) then I doubt that she would have told the truth to the relevant authorities when he was adopted.
To be honest, I very much doubt that, even if there was a baptism prior to adoption (and this would have been rare, unless the family were RC), they would have risked the truth at that either.
What age was granny/mum when the baby was born? What was her marital status? Have you had a look at the electoral registers to see who was living at the address where the baby was registered?