I can understand your concern for your fiancee. Since it has been for long and it neither hurts or pains does not mean you can ignore the problem. Your fiancee should get clinically examined by a surgeon. It sounds like a cyst.
If the doctor diagnosis
it to be a cyst. A sample of the fluid will be taken out in a biopsy
and sent for histopathological examination. If the result is fine the doctor will make a small incision and perform drainage
of the lump. He may give an injection for (local anaesthesia). Oral antibiotics
can be given. Your fiancee will get fine.
In case it is diagnosed to be a tumour. The malignancy of the tumour will have to be ruled out to continue the treatment. Your fiancee may need a biopsy, blood tests and if required some radiological examinations. Do see a surgeon. Have a good day, take care.