I work with a young 25 year old woman and she is beautiful and very sexy, even though she is 5'8" and 250 pounds or so. She's sweet, super smart, very witty, self confident, dresses and acts feminine, and loves being female.
its the complete person ... the look in her eyes, her smile, attitude, personality, how her body curves (not how thin or fat she is)
I would suggest that you tell her to stop paying to the glamour mags, that make it seem that women arent attractive unless they have the body of a prepubesant boy.
Brains and ability or willingness to help. If I am working on a car and am having trouble, a girl would be very sexy if she ran into the house put on an old T and jeans and dove in under the car with me to help. Thats HOT
Take care of yourself. Whatever nature gave you, take care of it and you'll be sexy.
For me, it always has been the total package, with her mind being the most important -- intelligence, humor, conversational ability, personality, etc.
I had girlfriends who were pretty much across the weight spectrum and across the breast spectrum, but it was always the non-physical attributes that got me.