Carrie,
  It is interesting that this woman sees cancer in her family
as a reason not to breastfeed. Ihave a friend whose mother had
cervical (or ovarian,I don't remember) cancer at one point and
 ultimately died of breast cancer. This friend had just the
oposite reaction. She has nursed each of her children for around
3 years hoping that the breastfeeding will offer her some protection
from her mother's fate.
  I agree with Carol, though. You never know what effect your
words will have down the line when she has her next child or
when she talks to a friend or sister in a similar situatin
in the future.  OUr job is to educate, support, inform. The
decision to breastfeed, how much to breastfeed, when to breastfeed
belong to the mother.
  Naomi Bar-Yam PhD