http://www.elevenplusexams.co.uk/prepar ... -in-year-5
See the link above. It talks about exhaustion and burn out. Everyone is exhausted at the moment I've read on here countless times about how tired DC can be at this time. I'm sure someone more experienced than me will come along to reassure and advise you. Have you talked to her about why she's now not performing as she did? I think I would agree a break with her. She knows the stuff now (otherwise she wouldn't've been getting 90%s before). I think maybe let her have a couple of weeks off and then start again, slowly, so that she can build up for the exam.
Have you asked them at AE tuition for their advice? I bet they see a lot of it at this time of year.