Probably monitored on the ground, since everything on the large social media web sites is available to government snoops, and then they alerted the flight crew and the landing crew. And I guess it's better that we not know exactly how they monitor, since terrorists can set off explosives with cell phones. He was dumb not to take up the matter privately with the airline instead of "pointing and laughing" on the world wide web.
IMHO, if he can actually do what he says he can do, the airplanes have a problem that needs to be fixed.
I find that frequently that corporate responses to problems is to ignore them or want them to just go away without really fixing them.
As I said in another post, the airplane industry (manufacturer/airline/etc.) needs to publicly prove this fellow wrong or if he’s right, they need to make sure the problem is completely fixed.
I would not want to be on a plane where any goon with a cell phone could monkey with the plane’s systems.
The goon might not simply have a sense of humor and write “Hello, World” on the passengers’ movie screens, they might do something nuts.
Of course, removing the passenger who proved the capability from one flight obviously does not do anything to prove that the vulnerability exists or does not exist, nor does it remove the vulnerability if it exists.