My mom only got an x-ray and a support-wrap thing to help her make it back to the US so I think it would all be A&E. But that was two years ago, she's been back several times and has no plan to apply for a visa so I think we're okay. But it's very good information to have, hopefully we won't need to use it though.
It's not just those who apply for visas to enter or remain. it is for anyone who tries to enter the UK when they have NHS debt. The UK want their bill paid and doesn't want to let them back in in case they are looking to run up more NHS debts.
The NHS trust must now tell the Home Office if someone has an outstanding NHS bills of 1k or over and haven't paid after 3 months. From April 2016, that was changed to an NHS bill of £500 or over and not paid after 2 months, See the link I gave above on that.
For your mother.
The A&E rules haven't really changed over the years, but not all emergencies and accidents are bill free. Anything that says things like "free for the over 60s" is not free, unless she had a limited leave to remain visa of over 6 months and had paid the £200 a year IHS.
If she had an appointment at A&E that she arrived for, then she pays. If it was something she had before she visited the UK, then she pays.
If she had an accident in the UK and visited A&E, then treatment there would be free if that treatment was received on the A&E ward.
If A&E sent her to x-ray (which is not on A&E) then that is free as A&E sent her straight there.
If she had been booked a bed on a ward and then went for an x-ray, the she pays. Once A&E book onto a bed on a ward, everything is billed, incuding any operations.
Aftercare has to be paid for.
If she used an ambulance, that was free for her. Treatment at the scene by a paramedic, was also free.
No life threatening treatment would be withheld, but it would be billed.
I never leave the UK without insurance, even on a day trip to France. All countries are different, even if the UK has a reciprocal health agrement with them and we often need private insurance too. In Australia, some the UK would pay under the that health agreement with Australia and my private insurers pay the rest (although I always get cover so that my insurers will pay it all). In the US I needed the same as your mother a few years ago and my private insurers paid just over $1,000 as neither the UK or US pay and these two countries don't have a reciprocal health agrement .