But she clearly struggles with time management, and I don't know whether that's related to her dyslexia, or to stress or what...
Yes it is, as PP have said.
Perfectionism (I could rant about this in students and myself and my colleagues for hours ...) I speak from my own experience here
otherwise why am I posting on MN on my lunchbreak?
It's a very useful smokescreen for a lot of other things. It can look very virtuous but it is a form of self-sabotage. It can also be a form of procrastination.
It is far far better to acknowledge that we can never be perfect.
Essays are a form of demonstrating your learning and your thinking. They're never ever an indication of your value as a human being. But too often young people - particularly young women - see their assessment grades as a summary of their worth. It is so sad.
Too often - particularly young women - are being pushed by our secondary education system to see anything other than "top marks" to be a "failure".
So she might need to readjust her catastrophic thinking, for example, if this is what she's indulging in. It won't help her produce her best work. Fear won't help.
She needs to think about her learning and her thinking - not her marks. This is something she could talk to her tutor about - I love it when students come to me with some ideas for their essay, before they write it, rather than afterwards to ask me why they "didn't get a First." We can have a proper discussion of their ideas and their thinking processes - which is, after all, what they're at university to learn & develop.
Finally, what is at stake for her in getting a First? If you set aside the erroneous/distorted view that a mark or a grade is about your value or worth as a human being, then why does it matter?
Unless she wants to go on to further academic study, such as a PhD, it really doesn't matter. And even then, the number of people I know who are highly successful academic, who didn't graduate with First Class degrees ... well, I'm one of them, and I'm a very successful academic.
So there's lots of really positive life learning she could draw from this experience.
Maybe try to help her reframe this as a learning moment, not a failure.