First, your Christian friend is an idiot if he really believes that the existence of ghosts somehow negates the possibility of God. Considering that a ghost is one of the persons of the holy trinity, then it seems like god and ghosts get along pretty well.
Second, given what we know of demons, no, you don't want to summon them. It's just a bad idea. Third, considering that humanity has been tussling back and forth about the existence of ghosts since the beginning of time, I'd say it's a pretty safe bet that they're not real. Thus, no one has actually had a genuine ghost encounter, they've only experienced things they can't explain.
I have summoned demons yes, know the rules upon summoning, casting a circle strong enough to hold one and don't ever believe a nice and fluff person to be not a demon. Ouija boards can be another dangerous device to play with. but ghosts do exist, some believe them to be an essence of something left behind, the person was so strong in life or they were so connected to something of possession that even in death they linger. and others believe they are actually true ghosts/spirits wondering about. Christianity is one of many religious beliefs towards ghosts/so forth, other religions and individuals believe otherwise. if it wasn't for so many near death experiences I believe I wouldn't have such a clear understanding of how this works.
How do ghosts disprove God? I though the idea among those type of people is that the soul didn't find its way to heaven or whatever.
Anyway: For me, in my path, ghosts are a natural part of the universe. I don't believe they are, for the most part, the spirits of the deceased, more like echoes of their energy left behind. I believe once in a while there MAY be an actual soul in the area, but generally not.
Yes, I can and have summoned demons. But I know how to put them away properly. And the word 'demon' is rather subjective, anyway, Christians have a very different definition than I do. And as for the why, well, because I needed something from them.
Hm, when I was a kid my room was really badly haunted. It was built across an old logging road. Really, REALLY freaky things happened. Doors opened and shut, things grabbed me from under the bed (literally- so many times that until I was an adult I slept curled into as tight a ball as I could so they wouldn't be able to get my feet from the foot of the bed), I had double doors on my closet and they would disengage from the tracks and slowly bloom open in the middle of the night...
The only really interesting 'new' ones are from quite awhile ago. A farm I worked at had been built over a civil war road, and we'd occasionally see soldiers walking down it. There was a lady (well, half a lady) who would stand at the top of the stares and occasionally try ad push you down, really creepy nasty thing. Then there was Fred, who would roll glasses down the hall, change the TV and alarm clock (I caught him at it one night after the third time he did it) and write on the wall- you'd see impressions of lines appear that would go away after you either read or traced them with a pencil. And he'd sit down on the bed, you could actually see his butt-prints.
Your friend sounds conflicted, but at the very least he doesn't believe in ghosts.
If I had the ability to summon demons I would because that would be cool as hell.
Yes, I do actually have an interesting ghost encounter. Unfortunately the encounter takes a bit to recount. I do not believe in ghosts. I find it more likely that my brain was convinced that I was seeing something rather than something actually being there.