My OH finished up with paranoid delusions as a result of a degenerative brain disorder (Parkinsons), rather than a mental illness.
If you are a partner, then it is possible that his professionals might be willing to advise you as to how to deal with this. They can do this without betraying confidences, but by simply responding to a question like: " When he thinks there are spies looking in the windows, do I go along with it and humour him, or do I challenge it?" These are the sort of practical things that relatives need to know.
I cannot begin to relate all the things my OH thought I might be doing. I can safely say that it was very stressful indeed day in, day out; night in, night out.
If it is not your partner, all you can do is be as supportive as possible to the family.
We found that medication did not help, but it may be because of the physical/structural source of the problem in my OH's case.
The only really helpful thing I was told was that, however freaky and totally mad his delusions were, to him they were the truth and to try and grasp how it feels to believe these terrible "truths." And that delusional thoughts are held much more strongly that ordinary ones. So, someone might be able to persuade me that I am wrong about something; but someone in the grip of a paranoid delusion is unpersuadable and totally immovable.