Exactly. We have a situation where the OP heard from her daughter, who heard from whomever at the Dr's office. This is information possibly passed on through 3-4 people.
The conversation could have ranged from, "You are dead wrong, Child has This syndrome, not That and he is on the wrong medication," to "I've noticed X in Child, have you considered That Sydrome, I had another similar student who had That syndrome and he did well on Z drug,"
I, too, get the emotional involvement, and it's hard sometimes to find a diagnosis and the appropriate treatment. Again, the bottom line is, legally, there is no HIPAA violation or any other violation. There is nothing actionable in what the nurse did.
Even if she did actually, "challenge" the diagnosis, she is still within her rights or scope of practice to do so.
She can't diagnose the child, but she is allowed to question a given diagnosis.