TL;DR
1. Don't sweat being scared. We're scared of the unknown. You're empowering yourself with knowledge.
2. Philosophy is about nerdy intangible shit. It's not physically validated making it all theories until it is proved or disproved with future studies limited by technological advancement.
3. Stigma = Fear. Fear ≠ Knowledge.
4. Boundaries are important. Try to find out from his family or someone who has seen him, what kind of boundaries he requires before dropping in. Everyone is different, some people want the company and support. (Some are introverts.) Also, be conscious of your boundaries with him.
It's okay to be scared. I struggled a lot with it, especially since my family has a history of schizophrenia. But it creates an opportunity for us to learn more and grow as a person. A way to challenge this fear is to inform yourself. We are essentially afraid of the unknown, a situation where we don't understand the situation.
Mental health is studied in Psychology. Psychology is a sect in Philosophy. Philosophy is the study of the intangible. We can observe emotions, but we can't consistently measure them clinically. Our understanding of the intangible is limited by our science and technology. This means we are perpetually learning more and evolving dependent on current or future studies and resources available.
Stigma is perpetuated by fear, which generally originates from a time when we didn't have the knowledge or resources we have today. The reptilian part of our brain is responsible for triggering one or more of the four f's of survival (feast, fight, flee, or fornicate) when we're placed in a situation which scares us. This is due to our adrenal system which kept us alive prior to dominating the food chain. It's also what gives people the strength to lift buses and shit whenever they see people in danger.
Stigma from a mental health consumer's perspective can be crippling. I used to facilitate recovery groups for people with mood disorders. It surprised me how many people limit their own personality by using a diagnosis for their identity. It's easy to do considering we were still performing lobatomies in mental health institutions as late as the 1970's. I'm really lucky to have known about Coursera when it was free. I did this course, and it opened my eyes to different aspects of mental health.
https://www.coursera.org/learn/mental-health
I think you can join and do one class free. That one would be worth the investment.
We also have a special part of our brain that can be used to trigger more empathetic responses to unknown situations. With our knowledge in neural plasticity, we can find ways to create habits that decrease the potential to trigger our reptilian centre.
Be scared. Own that shit, but don't let that fear define you or your beliefs. You're a smart guy, you're doing awesome by asking for more information.
When talking about unhealthy behavior think about the difference between enabling and supporting. If you're friend is experiencing psychosis but uses psychoactive drugs or refuses to take their medication (cuts cold turkey), make strong boundaries. Addiction is often comorbid with mental health concerns, and it can create caustic situations.
If you really want to support your friend, try to think about how you would feel in his situation. How would you want him to treat you if roles were reversed? Make sure those ideas wouldn't trigger him and go nuts. People just want to be treated like before when we have mental health symptoms flare. It's confusing and feels really shameful due to all of the preconceived notions of mental health. (Stigma sucks)
I have never been hospitalized for any mental health concerns. I have been clinically diagnosed with borderline personality disorder, but am high functioning. I don't wear my symptoms on my sleeve. I have a professional background in mental health. I have a warm, friendly, and inviting demeanor and create quick rapports with people. I experience psychosis because of the trauma endured in childhood. I was a SUPER sensitive kid. Cried at everything. And slowly I decided to stop reacting and start empowering myself. I've done courses, read multiple books, have years of volunteer experience with a crisis line that supports sexual assault/abuse survivors.
Super cool side story: one of my best friends has been diagnosed with high functioning schizophrenia among other ailments. He is unmedicated due to other ailments, but is able to consistently confirm what is realistic and what is delusional. Hands down smartest guy I know. He's going for his doctorate in Metaphysical Sciences and is constantly helping me with my thesis in the same field. Psychosis is a life changing thing, but it doesn't mean things are "done". :)