Here are just a few causes from personal experience:

Attempt to control at least some segment of their lives by controlling their weight.
Parents that were too controlling can lead children to become perfectionists who grow up worrying that their parents will be disappointed because their body isn't PERFECT.
Media of course is a big one for a lot of girls.
Fear of judgment, from family or friends.
Sometimes losing weight can be a plea for attention/help in other aspects of life.
Emotional/physical/sexual abuse in childhood can also cause eating disorders.

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There is no fast cure for this stuff, its a slow painful road that takes a lot of love and understanding from the people around.

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How skinny is "really skinny?" Height & Weight? And how old is this friend?