My friend really needs to gain weight plz help!!!!!!!!!!!!!!!!!!!!!!!!?

well my best friend is 5'2 and i am the same height as her and i am about 46kg (bout 100 pounds) yer i know im pretty normal but my friend is severly underweight she is my height (5'2) and 35kg (77 pounds) the other day she was crying bout it she has a high metabolism but she really needs to gain weight