Someone I know talks to an imaginary friend (this is a real thing I assure you not just for Halloween jokes)

I know a person who has been talking to someone that isn’t there a lot often. This person has been doing this a lot recently and whenever I question them they say they are talking to family and when I try to interrupt them they always tell me they can’t talk they are in a conversation. This person is very intelligent and they know me well but they always talk to this “person” weather it’s in a store, at home, or anywhere even out loud like yelling. What’s going on I don’t understand this.