This is a pitching fallacy that the pivot foot needs to be straight forward in the sprinters start. In fact, most of the elite pitchers actually turn their pivot foot slightly outward before they push off the rubber. This is a good thing. The difference is that Sprinters keep their shoulders square to the finish line when running while Pitchers are driving their body open to clear the body for the arm circle. Two different drive mechanics to achieve different objectives.
Never said the push foot had to be perfectly at 90 degrees in relation to the pitching mound. The OP asked why his DD was leaning or falling towards 3B.
There are some great example of pushing off the mound here: http://www.discussfastpitch.com/softball-pitching/5208-model-pitchers.html
The point is that her pivot is doing exactly what it should be doing and is NOT the reason for her body lean.
But when she seriously worked on her glove whirl and keeping her knee facing mostly forward, that hip snap look started showing up and her speed picked up. Provided her drive out was quick. Now, remember, I'm bad with words so this might not make much sense.
I dunno, I think 3rd base lean for a RHP is almost always caused by a desire to clear the hip and avoid any contact. There are a lot of 3rd base leaning pitchers out there, some are pretty darn good, but the common theme is a lack of brush. Encourage her to stay tall, keep the circle tighter to the body and let her know that a little brush is a good thing and the lean should go away.