This reply is purely based around the assumption he's gonna receive some sort of treatment.
Believe it or not, but the best person to ask about this is the treating doctor. There's a reason they ask for a very specific list of medications you are taking just prior to treatment commencing. Vitamins, drugs, oils, herbal products, natural products can all interact with chemo. Some can magnify those side effects you hear so much about, and some can even make the treatment less likely to work. Basically, you could end up killing him self-medicating or taking advice from some random on the internet.
They have probably heard this question a million and one times, so ask. There's nothing illegal when it comes to asking questions.
Since my wife was diagnosed, I have read some horror stories about people mixing non subscription meds/products. Just because something is "natural" doesn't mean it's not dangerous when mixed with other drugs. After all, the poppy plant is a natural thing and we all know what people can produce from that.
If you still don't fully take this onboard, please read up about Steve Jobs and his cancer journey. One of the founding members of Apple. Let the doctors treat him.
