This sounds weird to me.  There is no reason to believe there is
anything in milk that will cause thrombocytopenia.  Even if there were
something, the most likely thing would be an antibody or another sort of
protein which would not get into the circulation, or at most in only
tiny amounts.

The most common cause of thrombocytopenia in babies is platelets
sticking to the tubing because of difficulty getting blood.  This is
made worse by the fact that newborns usually have such high hemoglobins
and thus thick blood.

I think the physician is whistling through the hole in his hat.  He
should provide proof of what he is saying.  Which platelet inhibitor?
In the meantime, the mother should breastfeed.

Jack Newman, MD, FRCPC