I am a Christian & I believed there are ghost as that is one of the tricks from Devil!
But....what I worry is your fds might have some kind of Brian's sickness (e.g.: depression).
My husband experienced the similar case (saw something) & we found out that he had depression few months later on.
In human's brian have some "chemical" things & when it's un-balance, can cause "illusion". The worst time, my husband can even hear someone talked beside him (of-course, can't see la )
I will suggest your fd to have enough rest (if too tired) or do something to relax himself & see if that can help.
The more he/she think in that way, the more serious it will become.
I wish he/she will be back to normal soon & should he think that's really a tricks from the ghost/devil. He can try to pray to Jesus. It really help!