i'm worry about my friend

My friend always looks down when she walks.She always looks sad she does talk ,but when I tell her to come sit with me in lunch she refuses and sits by herself and doesn't eat at all.She's really skinny so I don't see why she doesn't eat.She also skips lunch in the restroom sometimes ,I ask her what she was doing a long time in the restroom.She tries to denies but I know she was in the restroom.She always wear sweaters and long sleeve shirts. One day I saw her arm she had scars I didn't ask her about it but I feel like she self harm herself.How can I help her be more social? and not self harm?