It doesn't necessarily require a medical professional to determine death.
It's been a while since I was crew, but as I recall, if you were performing CPR on an adult and there had been no response for 30 minutes (physically noticeable, or detected by the defibrillator) then you could make the decision to stop as death was evident.
If it was a child it was a bit different as you'd carry on CPR regardless (no matter what a travelling medical professional said) until handing over to ground medical services)