She has suffered severe migraines throughout her course and is also dyslexic (but has received extra time)
For a dyslexic student, she's done really well. But ...
She's already had mitigation for dyslexia. She can't double-dip.
If the migraines had an impact on her marks during her degree, why did she not raise the issue then & there? Most universities require students to tell us at the time when they think their performance has been affected by health or personal circumstances.
The way mitigation works at my university (she'll need to look at the regulations for hers) is that for specific time-limited conditions - such as migraine - we look at the results at around the time the student gives us documentation that they were unable to work at their best level due to their condition eg a migraine.
We look to see whether the achieved result is out of line with other results when not affected.
So the Examiners may well have done this (but not if she didn't tell them about migraines at the time) and agreed that the condition did not affect her specific mark, in relation to the overall run of marks. Maybe she is a student whose achievement is generally assessed at somewhere between 65 and 72 and overall, so the average (with weightings for Final Year "exit velocity" etc) is 69%. Close but not quite there.
Sorry, but she really cannot bring up a specific medical condition simply because she doesn't like her result. And there is no way she can question academic judgement.