Whys my friend so so skinny but eats like crazy??

My friend carmen is a size double 00 in junirs, super skinny, weighs about 95 pounds and is like 5'5... But i dont get it she eats like twice as much as everybody we know, and ive known her for about 6 years now, and shes always eaten like this and i know she doesnt throw it up because she never goes to the bathroom afterwards...she even trys to gain weight but she cant...she doesnt excercise nothing...so how is she so skinny??? is it normal or is there like something wrong with her, im just worried cuz shes been my friend since i was 8 (im now 14)