If your a true friend you want true friends to keep their opinions to themselves unless of course you want to lose your friend trust me
On the other hand, if you want to help your friend, instead of just making her feel bad then you should ask her to go out walking with you. Invite her to other physical activities. Do things with her that will help her burn calories.
Tell her your trying to get healthier and ask her to help keep you on a better diet by eating with you at lunch. Ask her to help you keep from eating bad snacks, eat more healthy with her.
Just pointing at somebody and telling them they are fat doesn't do any good.