Along with the bras I would suggest dressing "all in one color," and it does not have to be black. If she matches top to bottom it elongates the whole figure. It's a trick we "full figured" gals have used forever.
Another trick is to skip any type of cardigan, sweater, jacket, that falls open and constantly has to be clutched together up at the neck or at the chest. This draws attention to the whole top area. Instead the jacket/sweater needs to fall nicely on its own, or its a no go. (Lots of girls try the little sweater thing and then tug at it all day.)
Fitted jackets work well. For a young girl try jean jacket type fits. Those hang straight without clutching.
If she is not wearing them yet, shoes with a heel. It doesn't have to be a big ol' heel, even an inch heel will make a difference. It elongates the body. Flats make you like squashed.
Try hip belts slung low. Draws the attention away from the chest.
Big earrings are usually out. They draw the eye up, and make the body look shorter.
Fitted tops will actually look better than flowing tops. If the top is "flowing enough" to cover the chest then it flows over the hips and makes everything look bigger. If though, her hip area is larger as well, flowing works very well. Maybe a tight under top with a lightweight (key) flowing top.
No bulky fabric on top. Everything should hang not jut out.
If she likes short skirts wear them in the winter. Then she can pair them with black tights (leggings) and black shoes, again the long line.
If she has skinny arms go with jacket type vests in the summer over sleeveless shirts. Match the jacket to the pants, and wear a colorful top.
High hair. Draws the eye up. Long hair, especially if it hangs on the chest area, draws the eye to the chest.