Atheist, how do you define this?

One of my cousin was supposed to be caught or dominated (dont know the exact word) by a ghost) and was behaving very much abnormally. Everyone was scared of her. If anyone just thought about her, i mean just thought in their minds, she's look at them very angrily as she'd kill them. Finally she was cured by exorcism. What is atheist's about all this? Do they believe in ghosts? If not, then what all this is. I just have no answer to this claim.