If I die then I have a weak immune system

You know what is spooky.  I am reading The Skeptics Guide to the Universe right now.  I mean, I was before you mentioned it and last night I sent him some passages from the homeopathy section.  Clearly evidence of psychic powers.  

What if I formulated the problem like Affirming the Consequent?

If I have a weak immune system then I die.  He died therefore he had a weak immune system.  

Having a weak immune system can lead to death but it is not the not way to lead to death.  A person could eat the healthiest food in the world and exercise right and still be genetically predetermined to get cancer.  

There is a naturalistic fallacy

“I don’t want chemo because it a unnatural.”  There are unstated premises.  It’s unnatural therefore immoral.  It’s unnatural and therefore harmful.  He said he didn’t believe in “putting a poison in his body to heal himself”.  I am sure he sees chemo as toxins and that is what he believes caused his cancer in the first place.  That is in distributed middle.  

There are toxins in my body causing cancer

There are toxins in chemo

Therefore the toxins which cause cancer and the toxins which fight cancer are the same.  The “toxins” causing his cancer are his own cells and DNA.  

Appeal to Nature

Chemo is unnatural and Detoxs are natural so the detox is preferable and superior.  

This is why I don’t like homeopathic medicine.  It erodes confidence in science and in modern medicine.  Homeopathy kills people by turning them into nature worshipping zealots.   It puts thoughts in people’s heads that many forms of conventional medicine are unnatural and therefore bad.  You know what is natural?  Death...   Maybe living to 90 in unnatural.  Maybe more people would take their chemo if it was marketed as if it were natural.  

Lo and behold, I just looked up the ingredients of a chemo drug called Vincristine.  It is derived from plant alkaloids.  It’s a free range, vegan, organic, and all natural cure after all.  

I doubt providing facts to your friend will change his mind, and may even cause him to strengthen his erroneous beliefs due to "backfire." 

In my experience, people never change their beliefs by being punched in the head with facts. The reason is most people believe what they believe because they want to believe it. Facts are not important. Michael Shermer made this addition Cognitive Dissonance Theory in his book, “Why People Believe Weird Things.”

Peter Boghossian pointed out how to change a person’s belief. He said, to be successful, the person must reconsider how they arrived at the belief under discussion. If your goal is to change his mind, as distinct from pontificating (which is better done in front of a mirror), then you need to get him thinking about how he arrived at the belief.  

Instead of people holding a belief because they think they should hold that belief, you claim to hold a belief and wish you could stop believing—if only they could show you the error of your ways. This plays well with the homeopathic group. You want to get them thinking about the process to the conclusion, and not thinking about the conclusion itself. 

All of this deals with the Fallacy of Subjectivism. Subjectivism is not only a way of adopting conclusions on subjective grounds, but also — and probably more often — a way of evading the grounds. Some people have perfected the skill of ignoring what they don’t want to see, and most of us indulge this habit from time-to-time. Heuristics are hell. If I put the statement into a proposition, it takes the form: “I don't want to accept p, therefore p isn’t true.” That’s pure subjectivism.