No, a nurse does not have the authority to affect a patient's treatment. Only a licensed physician, PA, or NP (under a physician's oversight) can so direct the course of your treatment.
Where does MJ testing fit into this fiasco?
Can you please give us more information? Did you ever get to see the test? How do you know she threw it away. I take it you're in pain management. If you are then you know you can not have anything in your system other than what they're prescribing for you. Were you using marijuana, or just accused of using it?
Sorry for all the questions but the more we know the better we can try to help you.