This is a sinus which is draining serous fluid. It will require a sinogram, in which a dye is injected in the mouth of the sinus and x-rays are taken to see the extent of the tract. Once the extent and the depth of the disease/sinus is determined, a surgical exploration is done to excise whole of the tract. Healing takes place satisfactorily after that. Non healing of the sinus can be because of any suture material in the tract or tuberculosis which is impeding the closure of the tract.
Hope this helps. Please feel free to ask, if you have further questions.
Surgery should have been successful. It is a complication which can occur anywhere and in any surgery. But should have been recognized and treated far earlier then waiting for three years by her surgeon.