At some point in his life he held a seance to try to contact "the dead". Since he opened that door long ago he doesn't know why now they are showing themselves to him. I held a seance to conjure and see spirits and I absolutely did, lots of evil and ugly faces manifesting out of every shadow in the room. When I panicked and was desperate to do something I remembered what my mom told me a year or 2 earlier, that I can cast out demons in the name of Jesus. So I did that and it was gone and I felt immediate peace.

What I recommend your dad do is repent of having tried to contact the dead, conjur spirits, or do any form of witchcraft by asking Jesus Christ to forgive him for it, and all of his sins (very important) and to fill him with Jesus' Holy Spirit and finally, to make them go away, ask Jesus to close EVERY spiritual door that he opened by sinning against Jesus. I guarantee they will be gone and he won't even have to bother trying to cast them out in the Lord's holy name Jesus. Jesus Christ is God.