Maybe one/more of these options:
Talk to school about needing an extension on the internal deadline - Oxbridge and medicine actual deadline is not till 15 October, as others have said, tho the school obviously needs time to review the application and write the reference.
If still interested in medicine - she can only do 4 medical applications anyway, plus the UCAS-required non-medicine 5th application. For the 5th application, she could apply to a science course that she really wants to do (instead of the usual science/biomedical back-up choice, in case of no medicine offers). There will be problems of how to pitch the personal statement - universities are used to this, it will be clear that the applicant is a medical student making a 5th choice that they're less interested in. This isn't a problem for most universities, as they are generally keen to have these students, but it would be a problem for Cambridge Natsci as it's so competitive - a personal statement aimed at medicine won't get her an interview. Not sure about Durham. But doing it this way, she would only have one science option and might face disappointment if she doesn't get the science offer and later decides it's what she wants to do. Take advice from school - they might suggest this is bonkers if there's a real chance that she actually wants to do the science course, and she should wait until she's more sure of what she wants to do (really I think wait too).
Or consider Cambridge medicine, where the 3 pre-clinical years are more theory/science heavy than many medical schools. Yr 3 is intercalated and another subject (ie not medicine) is studied, with an undergraduate degree award at the end of the year - this could be a science (not sure if any sciences are excluded). There are also other medical schools that offer intercalation. Presumably BMAT and UCAT arrangements are already in hand if medicine was the original plan.
Or go for the science undergraduate degree, knowing that she'd be well placed to apply for graduate medicine afterwards if she wants to. Could just apply for Cambridge Natsci for now, and add the four other science courses later, before the January deadline. That would mean less pressure to choose the 4 others right now, just focus on the Cambridge personal statement. Make sure you're on top of the dates for the NSAA natsci entrance test.
Or forget Cambridge, and take time between now and January to make the decisions on science courses.
If really unsure, plan a gap year - maybe try some science work experience and science university tasters/open days. All university options will still be on the table next year, with the benefit of known exam results.
You say that she loves pure science. A good science degree is an asset in its own right - many graduate training schemes and careers (not all science-based) would be open to her.
I'd be concerned about medicine if she really isn't sure. It's a tough enough course as it is. With science, she doesn't need to know what she's going to do with it at this stage. If this were my child, I'd advise the science degree, or the gap year to think more about what she wants to do.