It sounds like it could be heat rash or a fungal infection.  The underside of the breast is a warm, dark place so rashes are common there.  Washing with a mild soap for sensitive skin, drying thoroughly, and using some dusting powder to absorb moisture may help.  She might want to try one of the powders designed for fungal rashes.  The drug store usually sells these in the athlete's foot care section. She should wear a soft bra that won't irritate the area further and be sure to take it off at night to let the skin breathe.  Showing the rash to your mother or other responsible adult would be a good idea since she will have more experience in deciding if a doctor visit is needed.  If a few weeks of these home remedies doesn't help, seeing a doctor to determine the cause of the rash would be a good idea.  Fortunately, skin rashes on the underside of the breast in a 15 year old are not likely to be cancer.