I have a friend that is 22 yrs 5'4 and 90lbs no one seems to say anything im worried what should i do or say?

she is really thin, im mean to the point that she told me she wears childrens clothes like a size 10 . and she never eats when i rarley see her eat she eats something very small like a jello and dont even eat the whole thing! im mean i know its her body and she is an adult but geez