As you well know, the problem with herbals and vitamin supplementation is that in general, we don't really know, one way or another. For example, if you google 'vitamin E neck and neck cancer' you will find a variety of studies with conflicting results. My husband's MO basically said no extra antioxidants DURING treatment. Since your brother is past treatment, it is probably not as critical, although the mutagenic part would flip me out.
I know this isn't much of an answer. Before his death, my dad (who scorned the medical establishment) took all sorts of stuff. I would carefully look up each new item so see if there was anything alarming associated with it. If there was, I would print the article, and give it to him, but at that point it was his choice, and my nagging him would have been counter-productive.
Take care,
Maria