A good article to review on milk allergies in breastfed infants:

Consumption of cow milk and egg by lactating women and the presence
of beta-lactoglobulin and ovalbumin in breast milk, Yoichi Fukushima et al,
American Journal of Clinical Nutrition 1997; 65:30-5.

In their study they found that 29% of women still had detectable levels of
cow's milk protein in their milk 9-15 hours after consuming 200 ml of
cow's milk.  The amounts of protein in the milk of the 24 mothers studied
varied greatly and was influenced by prior long term milk consumption.

Somehow I don't think giving the mother cow's milk will be a reliable way
to test the infant's ability to handle milk protein.  How will you know how
much or if any at all passes into her milk?  A true test of sensitivity can
only be achieved by direct administration of the milk to the baby.

Rachelle Lessen, RD, IBCLC
Children's Hospital of Philadelphia