but nowhere does it say nominee must be ap
In regarding the Nominee need to be the AP discussion, part of the requirement for the SE to enter the nominee info is the AP number. Hence, if the nominee is not a member of AP, they need to join AP first.
Right, you lock in a name and Aeroplan number, and you can only upgrade someone whose PNR lists that name and Aeroplan number.
So they absolutely must be an Aeroplan member.
However, the question is whether this could be changed after confirming the upgrade.
as per the thread that I quoted below, the answer is no, GA can't change it.
That thread talks about you upgrading yourself though. The upgrade system is complicated.
I have upgraded (or attempted to upgrade) people on my flight who do not have an Aeroplan number on their reservation, and that's fine.
So upgrading someone does not necessarily require them to have an Aeroplan number on the PNR. For the SE nominee, that requirement is more to ensure you can only have one nominee, but who knows if you can change it later.