It is tricky.
First thing I'd say is that when you are IN the city working in that sort of high pressure high prestige job you imagine that actually (a) this is normal, this is what people do, other people cope, you should too; and (b) this is incredibly important and exciting work, anything else would be extraordinarily pedestrian/crappy/dull/boring.
Actually when you are out of it, you realise that life in the City couldn't be further from normal - in the real world hardly anyone professional even picks up the phone after 7pm and thinks you are a nutter for trying - and there are a lot of interesting, intellectually stimulating and prestigious jobs outside the square mile, just with more realistic job expectations (ie you work really really hard for 8 - 10 hours a day, 3- 5 days a week, not the sort of 24/7 indentured slave existence that you get in the City).
It sounds to me that she is so overworked and stressed and panicked by it all that she can't see past the reality of her life to what might be possible beyond it. I can't stop working - I've spent so long training - I'm so good at what I do - I enjoy being good at what I do - I need to prove to myself that I can make partner - I couldn't do boring shit work - so I can't leave. That sort of thing.
But actually, there is a whole slew of things that you could do. You could do what you do now, but 3 days a week, say. You may not be able to do it in the firm you are in, you may not be on the same career trajectory (partnership will take longer, but you could do it. Or you could change areas of law to something that is less stressful. That is the PSL route, for example (which I could never have done - I'm a seat of the pants, instinctive, deal lawyer, NOT an academic in any way shape or form!) but it works for lots of people. Or you can do what a friend of mine did - she was asset finance or something and one day thought "fuck it". Chucked in her job, bought about 5 books and took a couple of courses in family law, and set up her own practice as a divorce lawyer. NOT brain surgery, requires a certain bedside manner, but it is a very self contained manageable area of law. She only deals with high net worth women (she gets referrals from her ex firm that doesn't do divorce), works 3 days a week, employs other women lawyers as consultants to share the caseload, pays them all about £200k and takes home £500k herself. You need to think laterally.
Or you can go into industry - lots of interesting jobs there, almost ALL with much better working hours etc. It is much better to be a client than a private practice lawyer from that point of view.
Or you can (if you have at least 7 yrs PQE) look at the judiciary. Don't laugh. Very keen on women, very keen on solicitors, very keen on intelligent under 40s with city experience. If you go the civil courts route they work in terms, so you get summers off, pay is OK, hours are 9-4.30, there is a career progression, and it is intellectually stimulating work which can be local to home (not necess london). Look at www.judicialappointments.gov.uk
I'm just saying it is a big world out there - this isn't working for you, so try and think of something that does.
Best of luck x