Yeah I agree, its not good for her, and could get alot worse...
If shes wanting to lose weight then she can excercise, eat healthy etc, not become anoriexic (although imsure this isnt her plan)
but you say she underweight, so she shouldnt be losing it anyways!
Deffinately tell her youre worried, or do something. She might not even realise theres anything wrong with what shes doing.
Does she eat evening meals?
Pushing food around on your plate is also a sign of an ed.
Good luck, and I hope shes okay.....
Its good that shes got someone like you to noitce. Alot of people go through this alone.