Another academic here to say that your DD needs to just forget about this exam for the moment, and retain her resilience and emotional energy for the rest of her exams. You need stamina to get through finals.
However, there are other marks in the bag, as it were, and so she will know that actually, these exams are not the only determinants of her whole future life and career, as people keep saying. I get strung out undergrads frequently obsessing about 2 marks etc, or "I really need to get a First,." No, you don't. Solid consistent achievement and learning not marks, is what you need for a satisfying productive ife & career.
It's important that those caring for 21 year olds eg parents, understand this and don't feed the hysteria.
OP if you or your DD want to be assured of the rectification which will happen, I suggest you go to the university's website and look for its examination regulations for her cohort (ie year of entry into the degree). There will be clear guidance in the exam regs for staff concerned with this incident.
As others have said at the Exam Board, academics will be absolutely clear that this mistake will have NO effect on the students' final degree classifications. And every university department Exam Board is formally convened, minuted, and attended by at least one (but sometimes several) External Examiners. These are senior people from another university who oversee a department's processes and ensure that students' work is assessed transparently, fairly, to a required standard, and in accord with that university's regulations. They will be told, and it will be minuted, about this incident, and it will be taken very seriously. The Externals may also comment on how their university might deal with such an incident. The Department Exam Board then reports upwards to a College or Faculty Exam Board - again, it is likely that if a whole module/course cohort is affected, this will be duly noted, and the actions to mitigate against any disadvantage to students affected will be minuted & actioned at the Faculty level. HE is a learning industry, and staff are invested in learning, as much as the students often possibly more so
I'll bet there are at least a dozen people all hanging their heads in shame - we do not like making mistakes and feel awful when we do, but we also aim not to make that mistake again. There will be an investigation of what happened, and procedures will be put in place so that it doesn't happen again. But I have to say, over a 25 year career, I generally teach & assess between 400 and 200 students each year, and up to 800 in some years (I work between a smallish humanities field and a very big one). I would mark at least two pieces of work from each student each year, so that's between 400 to 800 pieces of work each year, over 25 years = 10 to 20 thousand separate pieces of work in my career (of between 3,000 to 10,000 words each). And that's a very conservative estimate. In all that time, I have not once lost a student essay or exam script. So, you know, that's the pretty ordinary average standard of work academics do. Our ordinary standard of work is pretty high in comparison with the private sector, in my experience.
If this happened in my Department (although we don't have Finals in this way) we'd look at other marks in that module, and either adjust the marking scales or percentages, or keep the exam assessment in there, but award an average of other marks achieved in that module, if tis seemed a reasonable thing to do, in terms of student achievement. We'd then cross-check the marks for this module against all other marks achieved at that level (Final year) in other modules by each student affected. We'd probably also pull up stats from the cohort distribution of marks achieved in that module by past cohorts, to check that there isn't a huge dip. Most university's exam regs aim not to punish or fail students; most exam regs try to allow academics to judge in favour of a student's achievements. It's remarkably difficult to fail or even plough a Third, nowadays. Indeed, I often see threads in here, and public reportage about how we give "too many" Firsts, and Upper Seconds nowadays!
I think one thing that happens in these situations is that students almost fantasise that it was that exam which they were going to shine. Students often don't like us to award them an average of their previous marks. They also tend to obsess over a two or three percentage point difference, between their grade and a friend's, or between one assessment or another. In the broader scheme of things, these differences are not significant. And I often have to explain (I'm unusually numerate for my field, with a maths A level), the effect of arithmetic averaging on a wide array of numbers - they tend towards the middle.
Anyway, this essay is to reassure you OP that actions will e taken. Your best action for your daughter is to encourage her resilience. I do find students today really worrisome: very driven, very ambitious, but almost disproportionately anxious, dependent & whiney. It's a pity, as they are such wonderful, smart, energetic people. They need to realise they have agency! And need to develop agency. If a missing bit of an exam is the worst thing that ever happens to any of them, that is a charmed life ...