Student or Learner
In this case, are we supposed to use the past simple or the PP?:
"If you say this to her, she will say that you haven't understood/didn't understand a thing!"
A teacher, while teaching, will ask his students "Have you understood/got?" and not "did you understand".
Past indefinite is used when the matter is totally past.
In your sentence, when someone "will say this to her", it will be present at that time and not past.