I’m sorry this is happening, but I do think the correct decision is to retake the test. May I ask what the difference in her score was from 2nd to 3rd test? Just asking for knowledge as to what difference might lead to these cheating accusations.
You might also contact the ACT CEO Janet Godwin (her email is at the link below, and she has a history of responding to emails), because the delay in bringing these charges to students is unacceptable. To make these students retake the test in the middle of everyone thing else they have going on is just wrong and not student centered AT ALL.
Good luck to your D (and you.)