I wish she wouldn't be so conservative about food and start eating.
She is probably just naturally thin. I USED to be. At age 21 I started battling the Battle of the Bulge. My two older kids are naturally skinny. The 23 year old is 6 ft. tall and weighs between 120 and 130. My dad was just like that at his age. He didn't get fat until age 40. My daughter is 21 and has gone from a size ZERO jeans to almost a size ONE. I went straight into size 10 or 12 at her age.