How old are the twins now, 3? I vaguely remember that she didn't 100% leave the NHS, she put in for a career break. It's where you can be off for up to 5 years and come back to a job on the same grade, but not necessarily the same role. If I recall correctly (no longer work in the NHS/ wasn't in her trust anyway) you can't work for anyone else in that time, but you can be self employed.
I reckon she might go back in some capacity when the twins are at school, after that the option will expire and she'd truly have to job hunt with a big gap on her cv which might be tricky. Seems like Danny can cover everything for now, but she won't have much pension to speak of.