I would be fuming, hoping to take on some of that anger and action on behalf of my child, who has to pick herself up and move onto her other exams.
I would be looking for appeals processes, other information from the university as to what might happen of her behalf, as she is focused on her next exams. I might search website or call the university on her behalf.
And OP I would also suggest keeping in your mind that if you feel she has been affected in her other exams (has she "lost" time after the exam trying to find answers, sorting things, being upset?). Other students in her other subjects may not have been in this exam (not taken this subject at this time), so they come in in a better position, so to speak. Obviously if she's been expected to spend hours at student services, calling lecturers, being ask to provide docs then that will distract her from study and exams. If they are not asking her to do this, then hopefully her other subject exams won't be impacted greatly.
For those saying everyone will have the same impact, I disagree. Yes everyone in that exam had the same thing happen, but how they deal with it might be very different. The appendix/appendicies might have only related to some questions, and in theory other questions may have been able to be answered in full. But the exam moderators telling them there was no mistake may have meant students gave up/couldn't focus on those unaffected questions. Certainly that is bad exam technique, but we are human and being told something so blatantly unfair/untrue in an exam will result in some people making bad choices.
Not to mention other students sitting exams in her other subjects may not have been affected by this exam stuff-up, as they maybe they didn't take that unit this semester.
YANBU to be angry and trying to help her resolve what will happen. She is not able to spend the time getting this sorted, and knowing you are behind the scenes finding out things on her behalf might help her focus on her remaining exams. Hopefully they weren't sitting in a pile in someone's office and a simple error meant they weren't available to them.