The policy in USA (and most countries) is that once you call EMS, they take over the care, and one can not cancel a call (i.e. change their mind about no longer having a desire for assistance). So the nurse would have been protected: not only by the “samaritan” provision but also by the fact she would have been “following orders”. She could have argued that she called 911 according to protocol and afterwards they directed her to do so-and-so, which EMS can do even in a doctor’s office (let alone in an assisted living facility), until care is handed over to the emergency dept of a hospital.
I understand, as I have already written, the next available pretext would likely have been used to let her go.
In regards to “duty to act”, California sought to introduce that in the legislation, even for laypeople. This was in response to public opinion pressure, something terrible happened and was in the news for many weeks. It was not done because legal experts advised it would have been very difficult to enforce and would have punished only *unlucky* people among those who couldn’t care less about a fellow human’s life.
If some aggressive lawyers’ office contacts the deceased’s family and they smell the money and sue the nurse for refusing to follow the dispatcher’s orders, if this starts to get too many headlines and the assisted living facility fires her claiming she damaged their public image, it will not be me who sheds tears.