Fat necrosis in the breast is relatively common after breast surgery, like a mammoplasty. Sometimes the body can go in and just take care of it with no problems, sometimes the area will get surgically removed. Inevitably, there's a likely chance that scar tissue will develop there and will feel and/or appear like an abscess. It's basically one in the same; a lump of tissue. Certainly fat necrosis isn't an ideal outcome of a mammoplasty, but more often than not, it goes unnoticed and without symptoms because your body will remove that damaged tissue. I'm on the fence whether to define it a complication in this case because it's been so long since the surgery happened. I would see things differently had the patient needed surgery to remove it or if it had spread further than just a limited area. Regardless, it wouldn't be unusual for this length of time to pass before someone picked up on it. Usually they're found during mammograms or can occasionally be felt throw the skin on examination.
Needless to say, it would be a stretch to say this abscess (likely a ball of scar tissue) is a complication. The fat necrosis would have been the complication, but not the resulting abscess. In order to meet the requirements of classifying it as a "complication" there would have to be a clear statement of direct cause-and-effect in the documentation, which there is not. To make matters more difficult, these abscesses can come about from any number of ways... trauma to the breast, seatbelt injury even.
IMO, if there's going to be any connection of events, the fat necrosis would be the complication, if you could call it that, and I'd probably look at the abscess from a sequela standpoint, as you might do with scar tissue.
Nonetheless, I'd also query the provider about it.