Speculation on my part, but I would think it would be automatic. I researched this but could not find anything on the expected timeframe. It may be that your buyer really does not want to revise the feedback left, but does not want to tell you this, or it could be a glitch.
Give it some time and it may yet vanish, but if not, since you have communication through Ebay messaging that states the buyer agreed to cancel it, and that they said they did it, you could try calling Ebay CS for removal. You could ask them to read the messages between you that indicate she agreed to retracting the negative feedback and that she said she did it, and ask that they remove it on those grounds.
