G.B.
The reason to not treat her is that lice treatments is a pesticides and if she has a skin condition that is going to have this on her hair all the time you would be continuously treating her over and over and over with poison. So yes, you do need this diagnoses. Your doc has already diagnosed this. Give the nurse the note from the doc that says she did not have a single nit in her hair but that there was white debris in it. That is all that is needed. The nurse cannot go against a doc's diagnosis. She is underneath a docs status and cannot overrule him.
I don't know that I would be giving this this much attention. She did not have lice, the doc said so. A lot of people who have dandruff have shafts of white debris that slides off the hair too. It seems to me that it's a non issue.