I would say that she needs to be doing an hour a night schoolwork. I find it incredibly strange that she hasn't got a homework timetable.
She surely must have been doing a test/exam every time she completes a topic.
What exactly has she been studying this last half term? Macbeth? Then she should be revising macbeth.
She can spend an hour on her coursework for her main subject on Monday and Wednesday. Tuesday she can do an hours maths. Thursday and Friday she can do an hours English each night.
Does she have an online maths portal with exercises? Most schools do. She can spend one evening reading Macbeth. It is not a long play. She can annotate the text if she has her own copy. The second night she can spend watching the play either on iplayer or YouTube. Even better if she can watch more than one version. She will also find teaching resources on YouTube. Mr bruff has loads of resources including a quick, funny synopsis. Characters and themes and scene by scene analysis.
Now for July and August. She needs to reread whatever other text she should have already covered. A Christmas carol is only about 100 pages long, as is Dr jekyll.
She needs to find out what 20th century text she is covering. My guess is An Inspector Calls. She can read the play and watch the film during the summer. Again, I think the film is on iplayer. It is a very short play.
Its a bit difficult to revise the poetry if you don't know which you are doing. She will be doing 15 which are either from love and relationships, or, power and conflict, or a newer selection. Again, there are resources available online.
If she reads the texts in the summer she can do it at her own pace. She will be well prepared for the next year and there will be less stress next year.