Taking her to a doctor to have her legs checked out is a good idea. If the doctor is good they'll just have her hips x-rayed to see if she has a condition where the hip socket is to shallow causing this.
My daughter has one foot turn out like a penguin, the doctor had her hips x-rayed and said that the hips were normal, so then I took her to a chiropractor who adjusted her hips and her foot instantly turned in. Now all I do is watch her foot and when it starts turning out I take her back to have a treatment. By doing this her hip muscles will become strong enough to keep her foot where it is supposed to be.
I would have it checked out just to make sure there is nothing going on that shouldn't be and then work with her on stretching to keep her muscles from getting to tight.