Plasma cell mastitis appears to be part of the condition usually known as
duct ectasia. Usually the problem occurs in non-lactating women.
Was this condition diagnosed when she was breastfeeding previously? I wonder
if she is (or has been) a smoker?
I guess there would be some risk of developing a problem, but if she wants
to feed, I would say she could start breastfeeding and see what happens.
Very little showed up on a PubMed search, but this may be helpful:

Br Med Bull 1991 Apr;47(2):272-83
Non-lactational inflammation and duct ectasia.
Hughes LE
Department of Surgery, University of Wales College of Medicine, Cardiff, UK.
Many terms, including duct ectasia, secretory disease, periductal mastitis,
plasma cell mastitis, have been used in connection with a variety of
clinical conditions associated with nipple discharge, non-puerperal sepsis
and nipple retraction. The confused nomenclature reflects the uncertainties
regarding the singularity or inter-relationships of the main
elements--dilated ducts, periductal inflammation, bacterial infection and
nipple retraction. Recent clinical studies combined with new histological
and bacteriological information have set the scene for better understanding
of the pathogenesis and management of these clinical conditions.

Lisa Amir
MBBS, MMed, IBCLC in Melbourne, Australia

             ***********************************************
The LACTNET mailing list is powered by L-Soft's renowned
LISTSERV(R) list management software together with L-Soft's LSMTP(TM)
mailer for lightning fast mail delivery. For more information, go to:
http://www.lsoft.com/LISTSERV-powered.html