If your DD is actually panicked by the exam then preparing for that can be useful - rescue remedy and breathing exercises can help here.
What will she do if she opens the paper and there's a question she doesn't know how to do? How many marks is it? If its a couple of marks, have a go with the most likely technique.
If it's a longer problem solving question then:
Read the question carefully, underlining key words like 'estimate' or 'isosceles' or 'density' which might give a clue about how to start.
Will forming an equation help? What about drawing a diagram? Is there a calculation she could do and then see if that answer helps in any way?
If there is a diagram, especially for angles questions, writing angles that she has worked out onto the diagram helps see how she can work out other angles.
If a triangle is right-angled anywhere, think about Pythagoras or trig
If 3 adult tickets and 2 child tickets cost £34 and 4 adult tickets and 5 child tickets cost £50 then it's simultaneous equations.
Even if she can't get all the marks for a question, she might get marks for starting it.
Make sure she has the basics totally nailed and fluent. The formula for the area and circumference of a circle. Trig. How to find factors, multiples and prime factors. The primes, squares and cube numbers. Area of a trapezium and parallelogram. Exterior and interior angles of polygons. Angles on parallel lines (she should be able to tell an allied from a corresponding or alternate angle instantly). How to plot a cumulative frequency graph and a frequency polygon. Edexcel say there will be a lot of ratio and proportion on the papers (if she's Edexcel) so practice lots of different types of those questions.
And the main thing to remember with these papers is that she will find them hard because they are hard. They are designed with a large proportion of the questions aimed at 7, 8 and 9 students. She needs to go in knowing that it will be hard, but that she will be able to do some of it, and that the grade boundaries will be low enough to account for students like her who are not aiming for the top grades, who find the paper difficult. The average mark in the November mocks on the higher paper was 30%, so there will be lots of other students in the same boat as her.