What she does with her arms should not affect how much she loads. If she's relying on her arms to load, she's probably not loading properly right now. It's gotta come from the legs.
Here is a study done on long jumpers Role of arm motion in the standing long jump. [J Biomech. 2002] - PubMed - NCBI. I know its not softball pitching but it shows the power and balance the arms give you.
Cat Osterman, Monica Abbott, Yukiko Ueno, Jennie Finch, Angela Tincher, Sarah Pauly and Taryne Mowatt are some notable pitchers with an open backswing. They are among the most successful pitchers in the last 10+ years.