I've never had an eating disorder, but there was a point where I didn't eat breakfast (I never did anyway), and stopped taking lunch to school.  Came home, and ate small dinners.  My mom had left for about a week (my parents didn't get along), and I kind of just didn't bother taking care of myself.  Lol, when she came back after a week, she said I lost weight O__O.

Either way, it wasn't an eating disorder in my case, so it's possible it's not for your friend.  That doesn't eliminate the possibility of it being an eating disorder though.

You could always try talking to her about things and seeing if she has anything else on her mind. Sure she might not be eating in front of you, but that doesn't mean she isn't eating at all. Maybe she has a phobia of eating in front of people, and thus chooses to wait until she gets home. I know that most people at my school are like this. Makes lunch pretty dull. She could also want to walk everywhere in order to stay active and healthy. Nothing wrong with that.

she's still functioning. Which means she has to be intaking some food, even if it's a little. Just pay attention to her and see how she's doing. If you notice a drastic change, try contacting someone in her family whom would care about the situation. Hopefully she's fine :)

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hope shesa all good.

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yesterday was once tomorrow.