To quote Briggs and Yaffee "Drugs in Pregnancy and Lactation"

Diphenoxylate is a narcotic related to meperiding. It is available on ly i
combination with atropine for the treatment of diarrhea.
 The manufacturer reports that diphenoxylate is probably excreted into
breast milk and the efecffects of that drug and atropine may be evident in
the nursing infant. Once source recommends that the drug should not be used
in lactating mothers (Stewart JJ. Gastrointestinal drugs. In Wilson JJT ed
Drugs in Breastmilk. Balgowlah, Australia:ADIS press 1981:77. However the
American Academy of Pediatrics considers atropine (diphenosylate was not
listed) to be compatible with breast feeding.

In my humble medical opinion this means let the baby nurse and observe for
signs of sedation.  Take the dose right after a nursing session. That
Australian book may be why she was told last baby not the nurse.