AC's "preferred seating" is such a farce! I just flew YYZ-YUL on an A319. The first row of economy, supposedly ultra-preferred because one should have paid $25 or be an E/SE to get it free, was full on both sides. I was the only elite and everyone else apparently had no idea about paying for their seats...which of course the didn't. The gate agent just assigned them seats in this row. Meanwhile towards the back of the plane, low and behold, dozens of rows with empty centre seats!

As for upgrading, all you have to do is designate them as one of the four people you can upgrade (even when not flying with them). There is no need to link the PNRs. As for priority, if it is a summer flight it will be quite likely that lots of J seats will be available even for a non-status person. In fact, opups are very frequent on this and other European routes during the summer, but your friend would still have a priority over opups if he's waitlisted for an upgrade being sponsored by you. Even internationally, there is no real value to the so-called preferred seating on AC. If anything, it gets infilled first at the gate before the larger rear economy cabin. Just get yourselves two seats together anywhere on the plane and anticipate a eCredit upgrade.