I've had some success with companions on a different PNR. My advice is to call reservations and ask for the two to be "linked"* and if they can add the companion to the upgrade lists.
I've managed to get a non-status companion upgraded to First on a separate PNR for:
LGA-MSN: wife and I both cleared at the gate
MSN-LGA: work buddy cleared 24 hours out
DTW-YUL: wife and I both cleared into C+ at the gate (NOT on the C+ list - upgrades showed as RU)
YUL-MSP: I cleared at Gold window, wife cleared at the gate. She was on an award ticket - YMMV.
Technically, C+ upgrades are NOT available for a separate PNR but it has happened and it doesn't hurt to ask. Good luck!
*It's not really linked in a database sense. It's that they put comments in each record to let other agents know you are traveling together. So 1970s...