It's great that you can be there to help this mom find her way! She's lucky to have someone being suportive of her, and excellent that everyone is at home now :)
I have a few questions though:
What was the baby's birth weight, and weight when he left the hospital?
How was baby born, and what medications/fluids was mom given?
When baby was in the NICU, what was he fed? Did mom pump her breasts, and if so how often? If she did, did she make enough milk to meet her baby's needs or did the hospital supplement him with formula?
Now that everyone is home, how often is she nursing, or offering the breast? Is baby nursing at night as well or is he sleeping a several hour stretch?
Does baby have any other health concerns other than weight loss?
Has she seen a certified lactation consultant?
How important is a long-term exclusive breastfeeding relationship to mom? Often times the very answer to this question will allow a solution/plan to present itself :)