How can a teenager lose weight without exercise?

my "friend" thinks she's really fat and she's really tall. Like 5'10" tall and i keep telling her that she weighs so much because she's tall. Don't you think 155 lbs. is ok for a person who's 5'10". She wont listen to me. By the way she's only 13, so i wanted her to prove to herself that she can be skinny while tall. Do you have any ideas of what she can do?