could be a number of things, I would say go to a doctor and get it checked out, I know a couple of people that have had this happen to them, one turned out to have a cyst in her breast, but the other it turned out to be breast cancer. Now its most likely a cyst, or something thats nothing to worry about but you should always get it checked out, just to be sure.
Always get lumps checked out! Most are non-cancerous but still important to have a doctor exam her.
agree, she should get it checked. it could also be a blocked milk duct.