Yes, you say "maybe she had other compounding factors" which is equivalent to saying her case was idiosyncratic. In which case, why try to generalise it at all then?
When I look around the city, I see a lot of underweight women of all ages, including childbearing age — whatever the hell we're defining that as.* Many of them will be underweight by skipping meals, full on fasting, or eating and forcing themselves to throw it up later. This is the society, the mentality we have created.
Some of them I am friends with, others I have never met but write about it. Some claim to struggle with fasting, others fast and mention nothing about the difficulty or ease, and others struggle to eat enough. And some eat whatever they want and don't regulate it at all.
The takeaway is that when it comes to nutrition, there are a lot of factors and every case is idiosyncratic. There are no general rules other than "eat so you don't die", "don't eat so much you explode" and "don't eat toxic or corrosive chemicals". And even then, different people will have different tolerances to not eating, eating too much, and eating poison.
Nutrition is poorly understood, poorly communicated, and the driving factor behind the freight train of bullshit marketing telling people that they need to be wafer-like, elven creatures.
Full transparency, I'm underweight according to what I'm sure is actually a pretty shoddy measurement of weight...ness, but I don't do anything special. I fast quite a lot but I don't actually have a fasting regime, it's not planned, I just don't eat for extended periods of time.
When I do eat, it's really not a remarkable experience so I don't feel a need to change my behaviour. I eat enough to not die or suffer health problems thus far.
_____
* Since it will be a subset of {all ages}, my reasoning is covered.
