Genelle, I'm here on LactNet, Kathy Dettwyler, anthropologist who does
research on what would be a "normal" duration of breastfeeding for modern
humans.  The answer is somewhere between 2.5 years as a minimum and 7.0
years as a maximum.  You can find brief versions of my research on my web
site under "commentaries" (http://www.prairienet.org/laleche/dettwyler.html)
or you can look at Chapter 2 in Breastfeeding: Biocultural Perspectives,
edited by Patricia Stuart-Macadam and myself (1995, New York: Aldine de
Gruyter).  You can order any book over the internet from
http://www.amazon.com.  Just do a search on Dettwyler or the book title.  It
would be very nice of you to provide this mom with confirmation that nursing
a 2 year old is a normal and healthy thing to do.  The research that
suggests that prolonged nursing contributes to malnutrition fails to take
into account a very basic fact, which is that sickly babies who continue to
be breastfed survive in Third World countries, even if they look awful.
Sickly babies who are weaned too soon die, so they aren't around to catch
the eye of the researcher or to be measured in the height and weight
statistics.  I'll try to reproduce an illustration of this below.  I have
written some things on this topic but they aren't published yet.

Range of nutritional/health status of children 1-3 years of age still being
breastfed in a Third World context

[----------------------------------------|----------------------------------]
  poorly nourish, sickly            average for group             big, happy
  and not growing well,                                           healthy, smart
  but STILL ALIVE



Range of nutritional/health status of children 1-3 years of age already
weaned in a Third World context

[-----------------------------------]  [------------|---------]  [------------]
  These children are all dead.              average for group      No one up
                                            (higher than those     here.
                                             still breastfeeding)


Hope this helps you understand the situation.
Katherine A. Dettwyler, Ph.D.
Associate Professor of Anthropology and Nutrition
Texas A&M University