Well, that's the whole thing with Pauling... he advocated high-dose vitamin C as a panacea for just about everything (common cold, cancer, etc.) and he lived to the ripe age of 93. A lot of people have accepted that this is "proof" he was right.
As far as we know - let me say that again - as far as we know, Vitamin C serves two major roles and two major roles only. First, it is essential in the hydroxylation of amino acids during the formation of collagen. Without it, collagen cannot be properly formed (notice that the disease scurvy is directly link to dietary deficiency of Vitamin C). We, as humans and unlike many other animals, do not possess the biochemical pathway to produce Vitamin C. That's why it is, by definition, essential in our diets. Secondly, Vitamin C (a.k.a. ascorbate) is an antioxidant. Antioxidant's generally work by their ability to accept electrons from free-radical species. In this case, ascorbate can 'diffuse' the potential damaging effects of such free radicals as they float around.
Ionizing radiation, such as x-rays used to treat cancer, again - by definition - create "ions" that damage tissue. This is particularly important during cell division which is when radiation works. The DNA is irreversible damaged, and cell division cannot proceed. Since cancer cells divide faster than normal somatic cells, they are killed at a higher rate.
If you destroy tissue and have an excess of dead cells laying around, it makes sense that excess Vitamin C should prove benefical. First, fibroblasts will move in to lay down new collagen after the body has used the immune system to remove the debris. If you have a lot of new collagen being laid down, it's good to have a lot of Vitamin C to ensure that it is laid down properly. Secondly, the excess ascorbate can exist to scavenge and soak-up excess free radicals that the ionizing radiation may create.
It all looks good on paper, I agree. But, the problem is that large studies do not consistenly replicate this. I'm sorry to hear about your wife, but I'm sure there were a multitude of other factors at play and the fact that she stopped Vitamin C timed almost perfectly with a recurrence of her disease was coincidental.
-TT