I had the same problem after my first baby. I was around 50 kgs when I fell pg, got up to 72kgs then afterwards just kept loosing it. I got down to 7st 10. or about 47 kgs or so. At 171cm (5"7)tall way too thin. Anyway for me it was the breastfeeding that was making me loose it all. I don't think that I was putting in enough nourishment for the 2 of us.
Other than her eating a healthy and nutrious diet and maybe more frequent meals I don't know what else to say.

Hope your friend gets some help soon.