Fibroadenoma is still a bit unknown, like most of breast problems, the general assumption is that is related to an increse of estrogens. It is common before the age of 30 and it will probably go away the sooner she gets to menopause.
It can be followed by ultrasound but it usually is not associated with cancer. Some clinicians prefer to surgically remove it under an outpatient status with local anesthetic to test it to prove it is benign.