This second post was very revealing so thanks for taking the time to write. Your friend could be suffering from something called Delusional Disorder, jealous subtype. Here is a link for you to read more about it:
Delusional disorder is caused by an as-yet unknown genetic or biological neurological predisposition. If the individual then experiences particular biological or situational stressors, the disorder will emerge. You speculation that his drug use might be indirectly related is quite plausible. We know for instance, that in people who are at risk for developing a mental disorders such as schizophrenia
, that an extreme traumatic event or taking amphetamines can trigger their first episode.
There is some chance that if he visited the psychiatrist and agreed to take the medication prescribed, he would become calmer and his delusional thinking would moderate. Interestingly, something that might improve his demeanor and soften his delusions is if he quit smoking. Nicotine is a stimulant and aggravates delusional thinking in some people.
There is nothing you can do to persuade him that the beliefs he has about you and your mate are not true or irrational. He really needs to see the psychiatrist for the medication and he would do well to start visiting the psychologist again to help reduce his stress
level and assist him with quitting smoking----if he is willing to do this in the interests of his health.
I think you are in a fairly bad living situation if you have to fear that if you threatened to leave, your partner might beat you up. That would be terrifically endearing behavior, eh?
I hope this addresses your question. Let me know if I have overlooked anything. It was good to see your follow up post because it set my thinking straight.