Depending on why the infant is in the neonatal unit, specific food can have extremely deleterious effects. Disorders of metabolism are rare, but not as rare as you think, and most are controlled with specific diets. So if she purposely fed the wrong food, and in large amounts, it could absolutely kill certain populations. If an otherwise ‘healthy’ infant, the most likely cause due to overfeeding would be vomiting, and then aspiration of the vomitus. Infants, especially neonates, are very unlikely to be able to protect their airway. Even if a patient is already on the operating table, survival rates of aspirating vomitus is only 50% or so. That’s why you don’t eat/drink before surgery, btw.