Thanks for your question -
Such a dramatic change is possible but not without consequences that would make most plastic surgeons defer to treat her.
A breast reduction from a DD to an A cup is unlikely to leave enough tissue to keep the nipple/areola complex alive. This means the nipple and areola would need to be removed and put on as a skin graft. This would mean that they would be insensate, the patient would not be able to breast feed and some patients can experience depigmentation (changes in color).
In addition, it would be very difficult to get a good cosmetic result from such a large reduction.
A careful discussion with a plastic surgeon regarding why she would like such a large reduction should take place. The most common reason for breast reductions are to improve neck and back pain, reduce shoulder grooving and intertrigo (rashes and infections in the folds) and to improve shape. All of these could be achieved with a more reasonable sized reduction.
I hope this helps.
