Caren,
It may well be that this mom had less glandular tissue on her smaller side before her reduction, but this does not necessarily mean that she has so little that she will be unable to produce a full milk supply. Does the smaller breast look normal to you?  With the early soreness and weight loss I would go ahead and presume that the low supply is directly related to the poor nursing early on.  In my experience when there has been a large weight loss, the babies become less vigorous at breast so a very high priority is to feed (preferably at breast) as long and as often and with as much milk suppliment as baby will take by sns.  I would recommend using it at each feeding and within a few days the baby will be nursing well enough  (still with SNS) that additional pumping will not be needed.  A way to explain this to mom is that the babies suckling will improve more quickly with additional milk flow and by making up the lost weight and any catchup weight gain.  She must be quite discouraged, but your support and follow up will make a great deal of difference to this mom.