Students who don't know what they're talking about tend to just dig themselves deeper and deeper in a hole if they have more time. Students with (severe) hangovers would be hard to identify in an exam situation since many look bleary from all nighters or poor sleep due to exam nerves. Same applies to students with hayfever, heavy periods etc of course. If the hangover is impacting their thought processes, that will bring its own penalty.
I would design an assessment like this with time for normal expectations + maximum disability adjustment + further margin to remove time pressure for all concerned. Just going up to maximum disability adjustment could mean students needing the adjustment are under time pressure and others are not.
That said, a four-hour open book exam with two essay questions is already longer than most exams I've seen, so you would need to know the history of it. Was four hours already considered more than adequate for the task?
I also wonder whether the 25% standard adjustment should be applied to open-book exams - presumably there are different degrees of impairment with reading and writing, but is the support plan detailed enough to account for this?
I think universal design works well, and you can see from the above scenarios why it's appealing. But that doesn't mean the design can't be improved. I used to sit down with student groups, explain what I was trying to achieve, make space for them to come and talk to me privately, and end up with something we all happy with. But I always made sure they knew from day 1 they weren't being marked on a curve - made for a more constructive classroom.
I'd say again - the principle of them all having the same time isn't the issue - the issue is whether this student had enough time. 'Enough' is not x% more than everyone else - it's time to complete the task. She needs to get sight of her exam script after marking if possible. My students are often surprised when their longer, more detailed answers are less successful than something more concise and perhaps rougher around the edges. But we aren't looking for a certain number of facts or quotes or points or paragraphs at university level.
With this information, and some reflection on her approach to the paper, this student could certainly consult with disability support about any further accommodations needed.