I saw a few posts saying that this (the OP story) is part of the problem with buddy breathing, but keep in mind that the same thing could have happened on the octopus - where the donating diver just decided they are ascending too fast and turns fin-up and heads for the bottom. This is not a BB-only problem.
This reminds me of a real air-share situation I had in Redondo Beach in SoCal where I was the doner on a night dive at about 100 feet. This was no drill. We had buoyancy issues on the ascent, and while I swear that my BC was empty at that point, the other diver says the same thing. We did a (planned) 1 minute stop at 50', but missed the 15' stop and hit the surface inverted, kicking to try to keep a safety stop. I think he still had air in his BC. I'm sure he thought the same thing of me. In any case, we hit the surface.
There were no bad consequences, but it did trigger me to think that there has to be a better way to do this. I started researching what more advanced divers do. In that research I learned about the whole long hose/DIR route.
I'm not waying it's the only answer out there, but to me it made a lot of sense to have the divers ascending near each other but without the nose-to-nose death-grip that is usually taught. That way, each person is still controlling their own buoyancy, and if someone is too buoyant you know immediately who has the problem and needs to dump air. The person is still close enough to grab if help is needed, but since you've seen who has the problem you now know where to focus the effort to control buoyancy.
Also, ascending vertically as we did meant there was minimum drag to help slow our ascent. A horizontal ascent would have had more drag and been easier to control.
I've since been practicing many emergency situations and shudder to think what else could have gone wrong during that dive. For example, had a mask been dislodged during the air share the outcome could have been very different. It's usually not just one thing that happens, but a cascade of problems. Emergency situations tend to cluster.
As a footnote referring to the OP, when teaching rescue skills it is good to spend a lot of thought to make sure the 'emergencies' are happening in a controlled manner, lest they become real. I'm curious how the other diver did on the rescue course, though. Did they pass?
Ray