Dianne,
I think the most important thing you have done already for this mom is to
build a relationship of trust.  Now that she is successfully nursing, and
is reassured that her baby is safe from the dangers she imagined through
her pregnancy, I would focus on supporting her in establishing a good,
solid breastfeeding experience.  In a few weeks, when she is more confident
and self-assured, she may want to tell the office nurse how the unhelpful
comments made her feel.  I think that might have even more effect than you
trying to convey this message.
If this mother has time, keeping a journal about her feelings and
experiences through the birth and postpartum days may help her to
eventually sort out all these powerful emotions, though this may be hard
with a new baby and another child at home.  How sad that she had no one to
help her with her fears for her baby in pregnancy and at birth, and how
great that you are there for her now.  It is my experience that mothers
often have a need to review the birth and early postpartum experiences a
few months down the road, so I would try to make sure to contact her again
in 3-6 months if you don't stay in touch otherwise.  If she is able to go
on and continue in a happy and successful long-term nursing relationship,
that in itself will be the most enthusiastic endorsement of your
intervention.  Maybe that will be the way to best educate this office
nurse.

Anne Altshuler, rN, MS, IBCLC and LLL leader in Madison, WI