All that mother needed to do was take a (vegan) B12 supplement.
And check magnesium which I understand is necessary for adequate B12 effectiveness.
Nope. Most vitamins (including B-12) metabolize differently when taken as a supplement, rather than bound in food. Vitamin pills are almost all flushed from the system without more than a trace amount of absorption (which is why most supplements include way more of any vitamin than for the daily requirement... and still less than would be gained through normal food-absorption). This is one of the few “facts” in nutritional science that is exceptionally well-researched and pretty universally understood. If you’ve missed this in your research, you may not be as well informed as you think...