The odds of cancer are exceedingly low. The fact that this has been there since the onset of puberty without significant enlargement strongly argues for a benign lesion. The most common solid breast mass found in young ladies is a fibroadenoma which is benign but can enlarge when stimulated by hormones (like during pregnancy). Physical exam and possibly a breast ultrasound should typically establish a more definite level of concern.
take her to a breast surgeon or surgical oncologist.