"The first accuser, the lady Worcester, and Nan Cobham with one maid mo; but the lady Worcester was the first ground."
Historians aren't really sure who Nan Cobham was, and we don't know what testimony she gave to seal Anne's fate. The records of her trial were either intentionally destroyed, or Anne was convicted on verbal testimony that was never recorded.
There are several possibilities for Nan's identity. Retha Warnicke, in her Rise and Fall of Anne Boleyn speculates that Nan was Anne's midwife. Warnicke embraced the theory that Anne's last pregnancy was abnormal, and that the fetus she miscarried of was deformed, leading to allegations of witchcraft or sexual sin. However, there's no shred of contemporary evidence that Anne's fetus was abnormal.
Warnicke does make a valid point in one respect. The use of the nickname "Nan" indicates that the woman was "unlikely to be of high aristocratic birth," as Warnicke writes. This is true - if Nan had a title, Hussee would almost certainly have used it instead of her nickname. Aristocratic people might use nicknames in a casual conversation with one another, but it was unlikely he would have used it when identifying the woman in a letter, even if they were personal friends.
This would seem to eliminate one of the suspects: Anne Bray, Baroness Cobham. Anne Bray was recorded as one of the attendants at Anne's coronation, and her husband sat in Anne's jury.
Some speculate that Nan was the sister of Elizabeth Brooke (daughter of Baron Cobham) who had married Thomas Wyatt. However, I do not see an Anne listed among Baron Cobham's children. The only option in this scenario is Elizabeth's sister-in-law, Anne Bray.
A "Mrs. Cobham" is recorded as one of those who gave a New Year's gift to the king in 1534, and she appears in a list of Anne's "maidens," though that is not certain proof that she was unmarried. Anne Cobham, a widow, is recorded in 1540 as being given Warminghurst Manor for life, lands that had previously belong to the dissolved Syon Abbey. They could be one and the same.
Some speculate this Nan Cobham was the same woman who was named as being one of Kateryn Parr's serving women in 1547. If so, Nan retained her status at court even after Anne's fall, much like Jane Parker did.
Whomever Nan was, she doesn't appear to have been one of Anne's favorites or friends. She's mentioned on the periphery of the court, and appears to have been one of Anne's many serving women, appointed because of family connections or favors to supporters. But in the end, she became part of one of the most powerful conspiracies in history - to destroy a queen.