I really sympahise with anyone gaining such a close borderline mark, who worked hard and had feedback suggesting they were capable of gaining a first. There can be a huge element of luck and circumstance involved, at any time - which questions came up, whether some personal upset had recently occurred etc. But, every borderline student can't be marked up, that would be changing the border. Someone will always be in your dd's position.
Your dd will gain an excellent reference (and/or transcript, not sure how this works these days) which will make clear that she gained a 'top 2:1', which is a great grade and demonstrates consistent hard work.
But, maybe she really has worked consistently, the the best of her ability, throughout her course. Maybe she didn't have that extra capacity for excellence, when her enthusiasm and hard work all came together in her final year, required to take her to the next level.
It's really normal for students to gain a grade better in their dissertation than over all. I don't really understand your comment what I meant was that it was impossible to know the effect on her studies until the exams were sat and marked as people know when revising and going through past papers how well-prepared they are.
During my last course of study, only a few years ago, I did fill in an extenuating circumstances form for one year's exam. The advice on the form was essentially 'please don't waste our time with anything minor, the sort of circumstance we're interested in is serious medical issues or the death of a loved one (within a very specific timeframe, documented).' It is hard to see that stress, in someone who hasn't been medically diagnosed, due to circumstances that affected everyone, could come into this category.
There is a huge range of difficult, distracting and stressful circumstances that do have to be chalked up to normal life, or bad luck and taken in ones stride, however difficult this might be. Some people are more easily derailed than others. Learning to stay on track is part of learning to learn (and to work), hard as that can be.
As others have said, the markers will have considered the grade thresholds and cohort performance too. That should smooth out the 'strike effect', given it disadvantaged everyone on the same course, equally.