Descriptive word for someone who sees the world in black and white

Binary thinking seems like the term

Dent blog/website:

Let me introduce you to two ways of thinking:

1. Binary thinking – right/wrong, yes/no, good/bad, start/finish.

2. Directional thinking – moving forward, a step closer, lighter grey/darker grey, an experiment, an opportunity to learn, smart-ish, safer, right-ish, wrong-ish, finished-ish.

A generalizer

Generalize:

1. to infer (a general principle) from particular facts or instances.
2. to form (a general opinion or conclusion) from only a few facts or cases.
3. to give a broad or general character or form to.

Random House Kernerman Webster's College Dictionary, © 2010

Granted, generalize doesn't specifically reference two opposing categories. But I think it's a very pertinent term for describing the sort of individuals who reach very generalized conclusions about the world, rather than seeing the complexities and specifics involved. Black and white thinking is a very basic form of generalization.

Dichotomic thinker

Dichotomy:

1. A division into two contrasting parts or categories

American Heritage® Dictionary of the English Language, Fifth Edition.

Not a singular word, but still a term that means the exact same thing while condensing it into two words, rather than four.

naive /naɪˈiːv/ adjective –dictionary.com

lacking developed powers of analysis, reasoning, or criticism: a naive argument

It's naive to think that all things are black and white, and that one way is always either right or wrong. "Only the Sith deal in absolutes", is a very naive thing to say.

---

"but they stick to their own convictions."

steadfastingly naive

steadfast: resolutely or dutifully firm and unwavering. –Google

(according to dictionaries and my spellcheck, it's steadfastly, but that word rolls off my tongue into a brick wall)

Your steadfastingly naive friend is resolute, dutifully firm and unwavering in their desire to continue lacking development in their powers of analysis, reasoning and criticism.

I would suggest monochromatic thinker.

Manichean means “duality,” so if you or your thinking is Manichean, you see things in black and white.

Definition:

Manichean comes from the word Mani, which is the name of an apostle who lived in Mesopotamia in the 240’s, who taught a universal religion based on what we now call dualism.

If you believe in the Manichean idea of dualism, you tend to look at things as having two sides that are opposed. To Manicheans, life can be divided neatly between good or evil, light or dark, or love and hate. When you see Manichean, think “two.” -- source vocabulary.com

Examples:

"The most crucial feature of neoconservatism is its Manichean worldview, wherein the Earth is pitted in an urgent struggle between purely good and purely evil nations. As George W. Bush famously told then Sen. Joe Biden: 'I don't do nuance.'" --- Jacob Bronsther; What Do Neocons Have to Do With Obama?; The Christian Science Monitor (Boston, Massachusetts); Sep 29, 2009.

Paul was living in an exciting Manichean world of friends and enemies, right and wrong - in which he was the chief protagonist. --- BBC Apr 27, 2015

He could well be a Moral Absolutist:

Moral absolutism is an ethical view that particular actions are intrinsically right or wrong. Stealing, for instance, might be considered to be always immoral, even if done for the well-being of others (e.g., stealing food to feed a starving family), and even if it does in the end promote such a good. Moral absolutism stands in contrast to other categories of normative ethical theories such as consequentialism, which holds that the morality (in the wide sense) of an act depends on the consequences or the context of the act. (-- Wikipedia)