OP, sorry but this woman sounds like a real danger to all of you, unless the rest of you are always at least a couple of hundred yards before or behind her. Her actions could (as I assume you are all well aware) result in a motorised vehicle taking avoidance action to not hit her (or be hit by her - for her sake), and that avoidance action could mean that instead of them killing or maiming her, they plough into the rest of you instead, or another vehicle/person.
All of these normal human reactions can happen in a split second, as humans, faced with potential danger, most of us will have our brains hard wired to avoid the imminent danger, and just not have the time for our brains to work out that actually if we don't avoid this particular danger/person/cyclist, that person may end up dead, but the group of people/cyclists will not be maimed or killed. Of course avoidance actions by any type of motorist can also be dangerous/fatal for themselves, because they could swing themselves into the path of another vehicle.
Even if this woman was the only one to be maimed/killed, the rest of you will have either witnessed the event, or the aftermath very quickly afterwards, and that could be absolutely horrific and traumatic for all of you, and the poor driver of the vehicle. So I think that you as a group, and/or as individuals, are duty bound to tell her that she can no longer ride with you, as she obviously either doesn't understand the rules of the road, or just doesn't care. If you wish you can say to her that she will be welcome to join you again once she has shown you an up-to-date advanced cyclist proficiency certificate.
This is not being horrible to her, it could make the diference between at least one death, or even multiple deaths.
Cycling is a wonderful exercise, especially in the countryside, where there is hopefully much cleaner air to breathe in, rather than in an urban situation, and unless you are a mountain bike enthusiast, country riding will, by necessity, mainly consist of riding on narrow country roads. All any of us can do on the roads (whether we are on foot, riding a pedal bike or a horse, or in a motorised vehicle), is make sure that we are well versed in, and pratice, the safest way of doing our mode of exercise/transport.
Unfortunately we all need to be aware that any other road user may suffer from crass stupidity, and short of locking ourselves away in our own homes (which can also be dangerous - just visit any A&E department for a couple of days), we just have to lessen any particular danger as much as possible, whilst still being able to enjoy our activity. In this case that means follow your instincts, and no longer ride with this woman. Please.