This definitely needs to get checked out by a doctor
as soon as possible! You are right to be worried, as there are several serious medical problems that need to be ruled out. Your girlfriend should get in to see her primary care doctor
First, sometimes breast cancer can cause a sore on the breast. Usually, this is in a more advanced case of breast cancer that has not been picked up or treated in a timely fashion. Often there is an ulcered area in the middle of the sore and there may also be drainage.
Second, another serious cause of dark spreading spots is melanoma, which is an aggressive form of skin cancer which can present on the skin really anywhere on the body, including the breast.
Your girlfriend's primary care doctor will be able to examine the area and the surrounding breast tissue, feeling for lumps or other warning signs. Based on what they find, they may recommend other studies such as a breast ultrasound or mammogram, or a biopsy
of the area. These are the tests that can be used to rule out a serious problem.
Assuming that these tests are normal, your girlfriend's doctor will also be able to consider other more common and less concerning causes of the spot, such as inflammatory or allergic skin conditions. Good luck.