My Friend Is Suffering From Hallucination

Hey! My friend is seeing things that are not actually there (he does not do drugs). I don't see the thing/man whenever he says it's there. One time, when I wasn't there, he said the thing came and cut him with a knife and he was bleeding . How is that possible if the thing is only in his mind? He said that this man follows him around everywhere. That he just stares at him, and in his head he has dreams of the man hurting the people he loves. Also, he said the man is supposed to be himself. He said he feels like he has a lighter version of becoming a sociopath but he's very lovable and loving and I don't see how it's true... please help! I'm scared for him.