Helping a friend trying to kill himself

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I had this friend that I always get to see a lot. Last time he checked himself for psychological treatment because he said he is suffering from schizophrenia. He would tell me about hearing voices and other delusions that he keeps nurturing inside his head. i pity him that much but now I think im starting to get worried because he keeps telling me about killing someone whom he believes are demons trying to kill him. im trying to talk him out of it but he says as his best friend I whould be quiet. should I tell the police or just ignore him?