I'm another ex City lawyer. I agree that a history degree from Oxbridge and a GDL are classic ways in, but also that just getting your foot in the door is insanely competitive.
If she does get an offer she should take it as I agree it is a good thing to have on your CV. However, she should also apply to some of the good provincial firms.
The great thing about working in the City is the interesting deals, many of which will be in the newspapers while you are working on them. Yes trainees do get lots of dross, but they do get interesting stuff too. The stories about the long hours are absolutely not exaggerated and they continue after you are qualified.
The misogyny and old boys' networks are absolutely alive and well even now. You need a massive amount of stamina and mental resilience and many people drop out along the way.
If you don't want to work ridiculous hours for ever, once qualified stay clear of corporate, finance and litigation and to a lesser extent, commercial property. Having a specialism like pensions, trademarks or competition law is slightly less ridiculous hours on average but you will still need to put the hours in sometimes.
Remember all fee earners have a chargeable hours target which is the bare minimum they are expected to achieve (usually around 2000 hours pa but can be much more), and that 2000 is actual time billed, not time in the office, and to make partner you will need to significantly exceed this.
Ultimately all law firms are about making money so building up good contacts with people who will give you jobs is really the key and you can start doing this from a very junior level.
It is a hard and all consuming life and is largely incompatible with having children, but at its best it is interesting and challenging and teaches you to think on your feet. Intellect is important but commerciality is more so.
If I had my time again I'd have stayed in private practice until about 4 years post qual then moved in house or to consultancy.