My 115lb daughter wants to lose weight?

My teenage daughter is 5'10" tall and weighs a mere 115lbs, but she wants to lose weight. Right now, she has a floppy stomach and arms, and flabby thighs, and what I'm trying to explain to her is that she needs to exercise in order to tone her body because she's quite sedentary. She's so insistent on losing weight because she doesn't like the way the body parts look. What can I do?