I don't know if you've ever had much experience with metal illness, but it can be extraordinarily difficult to control or moderate the behavior of the afflicted.
No experience with metal illness as I've had my tetanus shots.
Seriously though, as you say it is very difficult to control his behavior if he is off his meds. Which is exactly why you do not let him go to the airport, one way or another. That is the absolute worse place for a mentally unstable person to be. She let him go into a situation with tons of armed security personnel all of whom are trained to take a six year old joking about a bomb as seriously as if it were really Osama talking about how he is goign to blow up the Statue of Liberty. In addition to the security personnel you have a situation with long tedious waits, cramped and claustrophic quarters on a plane, and other irritable people around and you are going to give all of those negative stimuli to this unstable person. It's just begging for a bad outcome. Now certainly you would have expected it end in a tragedy with the magnitude of death but other outcomes like arrest, detainment, involuntary commitment, or a bad scene are other things that you can also imagine could have happened under the circumstances. And based on the media reports it seems that she as well as the other passengers were aware that he was not dealing well before he got on the airplane.
Short of having someone involuntarily committed (which, given the state of our mental health care, is pretty much a horror show and almost no one would choose that if they felt they had any other options) or locking them up in their homes or drugging them into insensibility there really isn't much you can do.
Again, I'm not saying that she should have called the cops the moment he stopped taking his pills. Dealing with a mentally unstable person is one thing. Dealing with them in an airport situation is another altogether. I still say you simply do not take him to the airport if he has gone off of his medication and if he shows he is struggling beforehand then cut your losses and get out of there right away.