Concerned about a friend.

Was she aware that she had done something dangerous?

A friend of mine, now in her nineties and not suffering any kind of dementia, gave up driving in her seventies as she had twice noticed herself pulling out to overtake before realising there was something coming.

Whether this is a new development or not, your friend shouldn't be driving if she is doing dangerous things.

There could be other reasons.
Is your friend going through a stressful time? Is she sleeping ok? Even vitamin deficiency can have a detrimental affect on mental agility. (Vitamin B, I think?)

She was aware and quite shocked after the events but said she just hadn’t seen the vehicles. She is under work pressure a lot of the time but that’s nothing new and she’s certainly not done anything like this before.

It is a bit of a worry. My friend had a few near misses in her car, then an accident which she couldn't remember the details of. It was a few months later that she had a stroke.
Since then, we've wondered if perhaps she had been having mini strokes before, and not realised.

It sounds like your friend should maybe consult her doctor.

My friend had no health problems, however, at the time or for many years after giving up driving.
She just felt she couldn't trust her judgement in that area. I think she was very sensible.

Sounds to me as if the pressure at work is affecting her. Have you suggested she has her eyesight checked? If she literally did not see the other vehicles, either her eyesight as altered or she is too stressed or tired.

It sounds as though something is wrong - possibly a vision problem or the onset of dementia where her perception and executive functioning is affected and she is not able to "read" her surroundings and take acount of other traffic etc.

My mother had these problems when she developed Lewy Bodies dementia. We felt that her driving was getting to be dangerous - but had luck when she went into hospital for a planned operation and my brother "looked after her car keys" ... and forgot to bring them back.

With this type of dementia memory loss may mot be the main issue - Mum always passed standard Alzheimer's tests but she would sometime hallucinate and had difficulty understanding social situations and judge distance etc

from the NHS www.nhs.uk/conditions/dementia-with-lewy-bodies/symptoms/
As with other types of dementia, dementia with Lewy bodies typically causes problems with:
thinking speed
understanding
judgement
visual perception
language
memory (but significant memory loss may not occur until later on)

The most straightforward option is to talk to her, kindly, and suggest a general checkup with her gp mentioning these incidents. If she agrees you might go meet her after the appointment for coffee, to talk things over. Anything else is second guessing. Take it from there. She is lucky to have you as a friend. If she doesn’t agree to see her gp. ... that might suggest she is worried and then you may need to step up your support. A lot of guessing; one step at a time.

Thanks for advice/suggestions. I think I’ll try suggesting to her that it would do no harm to visit her GP. Other than that it’s wait and see and hope that there are no further incidents.