It matters when the winner noticed that the game ended in checkmate, and that the opponent's claim was in fact wrong. If the arbiter was the one to point this out, then he could defend his action in ruling the draw as standing by saying that he wanted to clearly communicate to the winner that the accuracy of the score is a solemn duty. It is, after all, why the player signs the scoresheet. By doing so, they are finalizing the result and implicitly waiving their right to contest it, unless new facts come to light.
Since new facts came to light, she now has grounds for disagreeing with the arbiter's ruling, and filing an appeal. She should learn her appeal rights, and the normal procedure for doing so. These include a hearing with the senior Tournament Director or Arbiter, and the possibility of an Appeal Committee it still not satisfied with the outcome.
I would certainly advise her to file an appeal, but it must be done within 30 minutes of the game's end (or the time of the TD's decision, if prior to the game's end as when disallowing a draw claim during a game), as I understand the rules.
Given that she doesn't seem to know when her opponent's king is in check, this may all be too much for her at this stage, however.