This seems an odd observation, maybe I'm misunderstanding you here. I'm sure a cop would be allowed to intervene if a nurse were being stabbed by anyone, meth head or otherwise.
As for the rule of law, well we are a nation of laws, it's going to take precedence. Unless in this situation suddenly the patient wakes up and proceeds to stab the nurse. Duh...
A 2016 Supreme Court ruling agreed cops can't draw blood without either a warrant or consent. But, this incident might be more complicated due to "implied consent". As explained here, though I do not know enough about interpreting law to know if this commentator is correct:
http://www.sltrib.com/opinion/comme...d-nurse-was-wrong-but-the-law-is-complicated/