Are they in their early 20's? I teach undergraduate nursing, and I am shocked at the general attitude that they should be given good grades for showing up, and that they generally ignore all constructive criticism. If your not telling them they are great, they don't want to hear it. It is one of my personal pet peeves that the younger generation thinks success should be given to them. Personally, I would talk to the other instructors to see how they are doing in their other classes and clinicals. I don't think there is anything wrong with giving a bad evaluation explaining that they aren't open to suggestions. With that said, the only thing you can do is try to keep a calm demeanor and explain that your job is to give constructive criticism, and that you are just trying to help. For the student who said it was a different diagnosis, I would ask them to define a diagnosis and a symptom. Changing the symptoms does not change the diagnosis, they are two completely different elements. You will have to make a judgement call on if you think the student is becoming a safe nurse. Nurses can have bad attitudes, but bad attitudes that foster unsafe practices should not be allowed to graduate without correction. It will make the school look bad.
I personally hate having to give bad grades to students, I would love to give all A's. I understand your dilemma because it is easy to give advice, and completely different to actually have to do something. If they are unresponsive to your advice for improvement, it should be stated in their evaluation. Students need to learn that job performance does have direct implications, and that there are no rewards without earning them.