I have many family members with mental illness, and a little personal history there as well. That said, I'm not a psychiatrist, psychologist, or licensed counselor. However:
The best thing you can do is help your friend to know he is not
alone. People with mental illness often feel a tremendous sense of shame and guilt, especially when family members reject them. Yet, it's not their fault – and it sounds like this friend of yours has plenty of difficulties besides.
The most important gift we can give to others is love – put another way in our modern context - that means validation. Validation that they're an important human being, not in general, but specifically to us
. Validation that their contribution to the world is important, and if they can't see it, we serve as examples of how. Validation that life may have its ups and downs, but that others, too, suffer and that they really aren't alone... even in a cosmic sense. Millions upon millions of people suffer with similar issues, and there's nothing weird or taboo about it.
For a person with schizophrenia, from personal experience and learned experience, I think the most important thing is helping them to understand that they are not judged and are indeed loved, not "in spite of" their mental illness, but because of who they are. They have intrinsic worth, and the mental illness does not take away from that.
Sadly, schizophrenia and related psychosis-inducing mental illnesses are highly stigmatized in our judgmental world. Therefore the greatest gift you can give (and it really isn't a gift if you're a loving, empathetic person, but just the natural order of things) is to both be yourself without reference to the mental illness AND to let that person(s) know that you're 100% supportive of them.
Now, for this person I don't know if they're taking medication, but as you may know that's a sensitive point for schizophrenia. I would encourage you to, if you and this person feel comfortable, allow yourself a role as an accountability person (as long as they are open to the conversation) to check in from time to time and see if they're taking their medication. Reason being – with medication, schizophrenia symptoms are alleviated to a great degree (though they may never completely go away).
And there is some hope on offer, too, though somewhat slim: schizophrenia can be grown out of, with the aide of healthy relationships, medication, and supportive care, over a lifetime. Yes, it's rare, but I cannot imagine a person with mental illness who wouldn't want to have, as their goal, a life free of it – no different than a person suffering a chronic physical illness of any other kind.
Now, not to get religious here – but I think the example of how Jesus dealt with certain people must be commended here, at least to you, if not maybe so openly to your friend. When faced with those in great mental and spiritual turmoil, Jesus did not turn away. All of the stories from the New Testament show an openness to facing it head-on, and indeed, without judgment. Notice, too, how Jesus was usually the only person willing to have that kind of relationship with those persons.
So we must also be open and loving to those who might suffer from challenging mental illnesses, because ultimately, we are no different than they are – no better, no worse – all of us dealing with our own life challenges in our own special ways, and not a one of us can really say that we are the better man or woman than those who struggle so.
Best for you and your friend. And what love you show already, by caring so much. I hope this helps.