The other emailed UK Border Force with a complaint, and a few weeks later she received an email with a full apology, stating that he Border Force officer was wrong in their actions, and that she was now free to travel in and out of the UK without issue for the foreseeable future. She has since been in and out of the UK around 4 times.
That doesn't mean that she can visit the UK as often as she likes because part of the immigration rules for visitors are V.4.2.(b)
"will not live in the UK for extended periods through frequent or successive visits,"https://www.gov.uk/guidance/immigration-rules/immigration-rules-appendix-v-visitor-rulesThere are many posts from those who had been granted a long term visit visa of 2/5/10 years, who then had their visa cancelled at the border when they try to enter again, for that very reason.
I don't recall any passport checks exiting the UK, so would going through the passport scanner be sufficient to proving you're flying out?
The UK have exit checks and know when people leave or enter. The airlines and ports record this infomation for the Home Office and they also send advance passenger lists to the Home Office, of those who want to enter or will be leaving the UK. Many EEA citizens who have ordered the details of what the UK hold on them, have been shocked to find the UK recorded each time they entered of leave the UK, as the EU says they must not have their passport stamped. The UK and Ireland also share information.
It's the same with those who have been granted a visit visa and who stayed longer than they claimed they would, but didn't overstay that 6 month visa. UKVI can see they didn't leave when they stated they would as the exit checks show that, and they then get refused when they apply agaiin. They might protest that they didn't overstay the visa, but unless they have a very good reason why they did not leave when they claimed they would and can prove it, they will keep getting refused entry because they were dishonest. e.g. were in hospital and could not travel on a plane even with with medical staff, but even then UKVI will check they they have not been using the NHS without paying.
It used to be very common to lie to UKVI/the border officer, show a return flight and then change that flight date when they have been allowed to enter. Too many people abusing, gets that stopped.