I suppose the relevant consideration would be whether the person is in a position to be privy to matters that are discussed with the Governor General--that is, a privy councillor--or someone else who.... It couldn't, I presume, be a person off the street; there would be a limit as to that kind of person being present.
My understanding is that it's normal for the Governor General to have access to constitutional experts. We are told, although no one knows for sure, that Professor Hogg was one such expert, and there may have been others. Although I don't know this, I assume that what happens is the Governor General consults with them separately. She may get up and leave the room or invite the Prime Minister to leave the room and then discuss that with them without the Prime Minister present. So they aren't actually privy to her discussion with the Prime Minister; they're only privy to her account of what advice she has been proferred.
Does that summarize your understanding of that?