All creatures including human beings are ruled by spirits. Spirits are separate elements. (A human being doesn't have a spiritual body.) A human being during his/her lifetime is living with many spirits which have joined one by one since birth. They are knowledge, skills, feelings, emotions, interests and everything. Even thoughts are not your own. For example, when you want to take a decision on a subject, one after another the spirits think and you just listen, choose or reject the ideas which they transmit to your mind through your brain in the form of thoughts. Brain is just a media to connect the spirits to your mind. A mind is just a computer's mind. After the destruction of a computer completely you will not get it's mind. The same is the case with the human beings. Soul is nothing but an energy needed for the functionality of a body. It is not a spirit or anything else. After death all spirits which accompanied a person quit and go to different places searching new bodies. No one lives in any form after death. All human beings are just robots made of flesh and bones and toys of the spirits for their games.
Sometimes some people are influenced by the spirits of the other people. Generally it occurs when they move closely with them. There is nothing to fear. Spirits are everywhere on this earth. People are often possessed by spirits either temporarily or permanently. These are all common phenomena. Your friend needs courage. A spirit cannot possess a person without his willingness. It needs at least a tiny reason. That is the nature of the spirits. They possess the cowards very easily. Therefore tell your friend not to fear. He must think and tell "This is my body and my mind. No spirit can possess me without my willingness."