I have been a Director of Nursing from quite a young age, and have been flying Long haul for many years, working in Australia initially.
There have been about a dozen times when I have been asked or a call gone out for different situations, on various Airlines.
Crews having asthma attacks, passengers having heart or epilepsy problems, or having had concussions then flown and had problems. Many flying related problems.
There is usually a Dr or Nurse on long haul flights, and if you volunteer, the Airline are usually very grateful, but it depends which Airline!
On KLM I have been loaded with gifts before getting off, a letter of thanks after, 10,000 fff miles, and and ipad for just the one flight! This was from LIS-AMS, when a guy in a matrial Art world contest, had seizures on the approach to AMS. He was very ill, and I managed to keep him safe without arresting.
Gulf Air of some years ago, gave me an upgrade voucher for my next return flight to Australa, which saved over £2,000! This for looking after a crew with asthma crisis for a few hours.
My worst experience, was a 5 yr old girl having a Grand Mal Epileptic seizure on a Thai 747 upstairs, as it was set to take off. We had to have her on the floor urgently as she was blue, and someome held us all as the 747 took off along the runway still, as we slid along the floor!
No thanks, at all from crews, but certainly saved a little life that day, as she was extremely ill, and went on to Hospital, as do most.
Airports often have emergency crews standing by for you, and have priority landings, immediate near gates ready, so much is involved.
Usually The Purser reports the incident, and a letter or thanks in whatever form is sent through the systems. If you are \ FF with that Airline it is much more likely.
There is no obligation to assist legally, in fact your insurers or professional body would warn of liability on your own up there.
Many of us do assist, within out competance level, which we will identify. Much is to do with calm, knowing the basics, and being able to re-assure and lead, in an emergency intervention. The limited space to assist, and conditions of flying often are frightening when you become or see someone be ill up there.
But if something did go wrong, you may well be liable in another Country or territory for your actions, outside that which you are registered or licensed to practice.
You are always asked for ID, and should identify yourself, firstly to the Cabin / Captain even if it is verbally in an urgent situation. After you complete a report for them and offer ID formally.
But that can be awkward if you are in another part of the world, and not Registered as a Dr or Nurse to practise there! It could have very serious legal implications if something went wrong or the sick person did not make it, you would be held to account for your actions.
Hope that gives a little more insight…