I checked out this issue with a rabbi who is very familiar with
breastfeeding issues since his wife is a LLL leader.

Breastmilk is considered a food - not a body fluid.  That is why it is
in a different classification than blood transfusions or organ
transplants.  Mother's milk is kosher and parve (not dairy/not meat.)
According to Jewish law, a mother MAY use donated milk for her baby from
any source.  However, in some very Haredi (ultra-Orthodox) circles,
the mother may not feel comfortable and by custom will not use donated
milk from a non-Orthodox Jewish woman.  If this is the case, she should
contact LLL or an Orthodox synagogue to find a mother willing to donate
milk.  By the way, Pikuach Nefesh (saving a life) is most important, so
if the baby's health is in danger, the mother may be willing to give her
her principles.

Susan Nachman-Srebrnik, IBCLC
Ranana, Israel
who learns new stuff everyday.