So original driver was sober, but had a head injury that was getting worse?
Second driver was drunk and not insured for the car? Were they insured on their own car? If they were, that might cover them for an emergency trip.
The last time I had 3rd Party F&T insurance (a few years ago from Elephant), it didn't cover general use of another car, but would cover driving someone elses car in an emergency. So if that was the case, he'd have been fine insurance wise.
As for being drunk, then he'd have to deal with that if the police got involved. And if they didn't get involved or suspect he was drunk enough to breathalise him, then they wouldn't be able to prove it unless they tested his blood-alcohol level within a certain time of the incident.
Maybe moving the car off the road onto the grass would have been a better option, you would have been seen on CCTV anyway and they would have dispatched someone to check it out.
You do the best you can and given the options available, you make the choice you think is best at the time.
Has the person with the head injury recovered?
Are the police involved?