When deciding whether or not to say something to a friend about her appearance or behavior, keep this rule in mind. If it is something she can fix in 5 minutes (something in her teeth, talking too loud, etc.) go ahead. If not, it is best to keep it to yourself.
This way no one has to hear, "You have gained a lot of weight!" or, "Did you know you have a zit on your forehead?". Obviously these are things your friend knows about and can't fix, so why make her feel needlessly self-conscious? And hopefully your friends will treat you this way in return.