I used to work for a recovery service, which was responsible for bringing home travellers who became ill abroad. My job was to escort the patient during his travel home. I was advised, by one of my colleagues, that if a patient expired whilst in the airline seat, the best thing to do was to place an oxygen mask over the patient, and have a discrete word with the crew.
Thankfully it never happened to me, so I was never placed in such a situation.
I can see the logic however. Its sometimes very difficult to tell the difference between a patient who is "nearly dead" and "dead", even to the trained eye.
Sounds a bit callous, but could prevent distress to all concerned ?