Nobody gets canned b/c they saw
a crime!!
You are probably correct. The nurse never actually saw Michael inject her with the insulin so she could not prove a crime was committed. I think she probably had enough reason to believe that there was something going on and she reported it to the director of the facility.
That being said lots of people get fired for making waves. Also Michael IIRC has a nursing degree and may have been quite friendly with the director.