Haven't read all of the thread, but...
Whilst I agree that it isn't really your DD's decision, and she should still sit the test, I'd be making very sure of the culture of the school before I sent her. It's far from all of them, I'm sure, but some of them can be real pressure cookers.
I went to a super-selective all girl's grammar for sixth form only and was fine - but I was shocked at the state of the girls in the lower years and the lack of pastoral care and concern given to them.
If your child is a robust, confident character and/or is so academically able that they can pass the entrance exams with flying colours and will NEVER struggle, then these schools are, or can be, a great boost.
But, for those who aren't that type, who sit on the edge of the ability needed and/or who are nervy or anxious or worriers, the school I went to was an utter disaster. Great results, fab Uni admissions - but, my God, the issues those girls had, none of which were supported, because nothing mattered more than the results.
I saw more eating disorders, stupid piercings, careless, meaningless sex, tears and stress-caused illness in two years at that grammar (and it was one of top performing non-fee payers in the country) than I had in five at my bog-standard comp in a not-great area. Every single year had a notable cadre of the ones who couldn't cope for whatever reason and it was horrible to see. These were bright, brilliant young women with great potential being destroyed by the environment day by day, and every time they cracked a little, it made it harder for them to keep up, which upped the pressure and so on, and so forth.
From the sound of things, from what your friend is saying, this school is similar, so be wary, OP. A great education just isn't worth your DD's mental health. She has her whole life to go back and resit exams.
I'm not saying don't send her, but listen to what she's saying and how she's reacting now, look hard at the school and keep an eye on it, please. She has to thrive to be successful, whatever who school results.