but I'm not sure how to assess whether she is up to the exam
You should seek the opinion of her current teachers to get an idea
whether its worth putting her through all the stress of the preparation for it
The requirement is to be familiar, as much as possible, with all the question types and exam format. IMHO 1 or 2hrs per week in the six month previous sounds more than enough. Ideally they should find it fun and challenging rather than stressful. Working in a tutor group is quite a good idea as they see their peers are doing the same thing and can develop a healthy competitive attitude.
I'm hearing of girls in her class already having 2 hours of tuition a night to prepare them for the exam!!
IMHO 2hrs a night is madness but not unsurprising given the obsessive approach taken by some.