Ok, I get that the form wasn't done, but just as a randome musing - why on earth wasn't a DNACPR form done before extubation in a patient who is absolutely expected to die within the next 2 hours? Plenty of time to do it before pulling the tube out!
To me, the situation is pretty clear - a consultant has established (no doubt via talking to neurosurgeons) that The brain injury on CT is not survivable. That's pretty concrete. As a result, CPR is futile / not in the patient's best interests. Usually when I / others talk about DNACPR to relatives, we make sure that they understand what it means, and that other rx will be given up to the point of the 'heart stopping', and if needed I gently explain what a resus team does and how it would be undignified. Armed with those facts most people would say "don't jump on granny's chest and break all her ribs".
We know that she will die within 2 hours and that nothing can reverse that. I would still therefore say that CPR now would be futile and inappropriate. However a lot of this rests on being clear yourself what the discussion was that happened between the consultant and the family.
The other option, of you really thought it appropriate, would be CPR whilst some narcan / flumazenil was given. But I think it would be difficult to justify CPR in this patient's case when she has been extubated to die.
However, I'd reiterate that as a junior if you are unsure in a CPR situation, start CPR. In this situation, it is going to give very conflicting messages to the family though. The other option, in an ITU unit, is to pull the red button and people will come running within a few seconds.
I'm a bit confused by you refering to the drugs "keeping her asleep" though. It says she came in unconscious.