Janice:  Please tell this mother that the theory about lack of menses
being the protective factor against breast cancer is just that, a theory.
I never saw any data about return of menses in the studies which
showed a reduced risk of breast cancer, although breastfeeding mothers as
a group get their menses back later.  That's why researchers often mention
the delay of menses as one of the theoretical reasons for the association
found in their studies.
However, the study among Chinese women who breastfed unilaterally and had
significantly higher rates of cancer in the unsuckled breast strongly
suggests that there is something about suckling in and of itself which is
protective, irrespective of menstrual cycles.  This mother should continue
to nurse her child not only because the studies show reduced risk with
longer duration of breastfeeding, but because of all the other important
benefits she and her baby can continue to derive.  If her baby is a girl,
she can feel especially happy that women who were breastfed have lower
rates of breast cancer themselves (independent of whether or not they
breastfeed).  But she must remember that reduced risk doesn't mean no
risk.  Her genetic predisposition is a factor which she cannot change.
Breastfeeding is the only thing which is in her power to actually change
the odds in her favor.
If she wants references for the studies I mentioned, she can e-mail me
privately.  Hope this helps.