It would appear that your father is not suffering any damage that would appear externally.
He is very fortunate, but much like the Fluoride in the water, we don't see the long term damage without looking inside our bodies. With that said, you didn't say that Amygdalin wouldn't work. So wouldn't it be more preferable?
I don't know anything about Amygdalin. Here's the cold truth: when an oncologist says "if we do chemo, your father has an 80% chance of survival; if you don't do chemo, he'll die within 2 years," then you choose to do chemo. Long-term effects be damned.