Parasitic infection can be a problem.  I had one child (age 2 years) in my
practice who developed Giardia lamblia (her mom got it, too) and subsequently
developed such an aversion to eating that her weight fell way off the growth
charts.  She would nurse barely enough to keep herself alive, but was
essentially gaining no weight.  We finally had her admitted to the feeding
disorders program at the Kenndy Kreiger institute in Baltimore.  If the mom
you describe feels her baby isn't well, then she probably isn't.  Weaning,
however, is not necessarily the answer.  The child does need to relearn to
eat appropriate solid foods for her age.  Also, it is essential to make sure
the parasite is cured, or the child will continue to not feel well and the
lack of eating may get even worse.
Linda L. Shaw MD