Folic acid is required for stable DNA, something that cancer doesn't need at all (it needs mutations for plasticity).
Its also needed for DNA production, but many more cells need it, those that have short lifespan - cells lining the stomach for example, sperm cells, some immune cells etc.
What you're saying about stable DNA makes sense. I'm curious, though, if the supplementation helps the body generate more NAD+ (relative to someone with a deficiency), and if that could help cancer proliferate in its earliest stages (hence why the correlation is most pronounced in people already likely to develop cancer--smokers).
NAD is essential for many functions, especially longevity as sirtuins require it for proper function.
As far as I know only massive Vitamin C doses uniquely influence cancer in negative manner and get pretty much no effect on non cancer cells. Massive vitamin C infusions (which work via mechanism similar to chemotherapy) while can't cure cancer per se, have potential to make life span and quality of life much longer (toward chronic disease).
Is this why some people think fasting can help? It gives the body a chance to clean up cancer cells, since you have more time in a catabolic state?
- No sugar input => low insulin (insulin promotes cancer)
- No nutrition for cancer cells => no folate etc.
- Metabolic slowdown due to starvation => means everything works slower including cancer
- Promotion of autophagy which consumes damaged organelles. This one is dubious as cancer cells can do autophagy themselves to stay alive, but I suppose that due to extensive mutations in cancer there is still a chance that normal cells do it more efficiently. In healthy parts of the body this can also lead to better immune system as damaged parts of it could be replaced in the process. On the other hand, good immunity needs various vitamins and minerals most of which can't be produced or stored.
At this point there is literrary nothing you can recommend to your friend.