Hello, thank you for writing us today. I am sorry to hear that your friend is experiencing this. As you are aware, hearing voices (hallucinating) and being paranoid may point to a condition like schizophrenia, although the person should be fully evaluated/assessed to get an accurate diagnosis. Often, medications are utilized to treat the hallucinations and they can be hugely effective in helping a person.
The best thing that you can do for this person at this point is to try to "normalize" that others have similar problems and do well when they get help. This is why there is a huge field of medicine devoted to mental health - mental health practitioners like psychiatrists, psychologists, social, workers, etc. manage patients all day long who have similar issues. Therefore, you are explaining to them that they will not at all be treated like they are crazy. Yo can also try equating mental illness to physical illness. People see doctors and rely on medications for all kinds of health conditions- diabetes, blood pressure, cholesterol,. etc. so taking medicine to help with optimal functioning of our brains is no different. Also, explain that they may not have to live their life hearing these negative things - there are medical interventions that can help. If you are in a position to do so, you might consider offering to make or just making an appointment for this individual and "insisting" they come along. Make it very easy on them to do so - offer to drive, offer a lunch date after, etc. Sometimes that nudge of having an appointment scheduled along with your encouragement and companionship can be enough to get them to go.
Lastly and unfortunately, if your friend remains resistant to seeing a doctor regarding these symptoms, there is little you can do other than offer support and continued encouragement. They are the only ones truly in control over their behavior and choices so despite your best efforts, they need to be in charge of walking into that doctor's office. Unless they are a threat to themselves or to someone else, there is little power you have to force them to get the proper treatment. So, you remaining supportive and encouraging may ultimately help empower them to get help, but this is a decision they have to be willing to make.
Best of luck to you and your friend! Please let me know if you have follow-up questions.