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These can be skin splits from dry skin. If they aren't too bad, you can wash her feet well, and apply NuSkin (from the drug store) to help the splits heal. Then she can apply lotion to her feet to help moisturize them.
It's difficult to say if this is the cause or not without actually seeing the feet. So you may want to take her to a walk-in clinic for a diagnosis to make sure this is it.
Of course, she shouldn't go barefoot until the sores are healed.
Please let me know if you need more information.
Like I said, it's difficult to tell what it is without seeing it. There may be a reaction to something in her shoes (a fairly common thing) that is causing some irritation, resulting in skin splits.