I suspect this case will hinge around whether Mr. Armstrong took off in the helicopter knowing that he would be required to take over the controls on arriving at the scene due to his co-pilots' inexperience.
If he went along intending merely to advise and assist but ended up taking over the controls without any intent to do so when the flight took off, then that is another matter. That would seem to fall into a similar category as to when an unlicensed pilot or passeneger lands a plane after their instructor suddenly becomes incapacitated.
In genuine life or death emergencies, actions can be taken to save life which would not normally be acceptable in law. But there will need to be a court case to determine whether Mr. Armstrongs' actions were reasonable at the time taking into account all the circumstances. Even if found guilty, the court may decide to impose only a trivial penalty reflecting the successful outcome of events.