You may try an over-the-counter antifungal cream like 1% clotrimazole. Your daughter should also maintain hygiene and try to keep the area clean and dry, wear loose-fitting cotton clothing. She should use an antibacterial soap, avoid deodorant / cosmetics.
Since it is impossible to diagnose skin conditions without actually able to examine the patient, I suggest that you consult a dermatologist who can determine the cause and prescribe appropriate treatment for it.