Can you clarify a couple of points?
Is the "birth certificate" you mention your friend's father's actual, original birth certificate in his birth name, or the adoption certificate (which, in its short form, looks identical to a short form birth certificate) in his new name?
Am I right in understanding that the mother's name on the certificate is that of the person knew as his grandmother?
Adoption records can be made available to the adopted person themselves but not, alas, to anyone else (including descendants). Is your friend's father still alive?