Tell him he’s wrong. It was the Umbrella Man. The umbrella had been used to fire a dart with a paralyzing agent at Kennedy to immobilize his muscles and make him a “sitting duck” for an assassination. Refuse to budge from this.
I’ve never argued with a JFK CTer. But, just for my own information, I’d sure like to know why, if these theories are “facts”, are there still so many alternate “explanations” of who was behind it?
It would seem that a large majority of the CTers would have converged on, say Castro, by now if there was any substance to their claims.
You can kill almost CTers ideas by asking them where Connally sat WRT to Kennedy. If they say to JFK’s right or directly in front, show them the many pictures, etc. (as well as the blueprints from Ford, etc.) that show he had to be to JFK’s left. That throws out the whole “magic” bullet nonsense. This particular garbage was first published in 1964 and immediately disproved but virtually all of them still believe it.
Hear them out, then say, “That’s just what the Illuminati want you to think. …”
A comedy show I used to watch did a sketch with a fictional game show Who Killed JFK Today? The objective was to explain your conspiracy theory as quickly as possible. The champion held the very reasonable theory that Kennedy was killed by a tiger.
I’ve posted the video before in Kennedy conspiracy threads. I don’t know if it would dissuade anyone.
As always, the Onion definitively answered these questions decades ago - it was everyone all at the same time.
One can find articles and podcasts saying that the earth is flat, that various public figures are lizard people, that we never landed on the moon, and that there was an alternate ending to the film Big. No matter how batshit crazy or unfathomably stupid a theory is, one can always find proponents of it out there, claiming to be the Keepers of the Truth. And one can always find other people gullible enough to believe it.
You can’t reason your friend out of his position. You could point him to various JFK CT debunking sites (I haven’t actually looked but I assume they’re out there) but I wouldn’t hold my breath for a change in belief.
I think a certain author started the lizard people as a way to mess with people, not really meaning it.
And a certain other author started a now-popular “religion” to make money, not really meaning it. Doesn’t mean other people don’t now mean it.
I would simply say, “I’ve spent years examining all the evidence available to me, and I’m convinced that Lee Harvey Oswald acted alone. I know you’re just as certain that there was more than one gunman. At this point, there’s no new evidence that could change either of our minds. Arguing isn’t going to get us anywhere, so why don’t we switch topics?”
“JFK would be dead by now anyway and so would anybody who shot him, so case closed.”
“You should care because the forces that covered it up 60 years ago are still pulling the strings! Just look at this website and…”
“Well good news! I’ve got evidence you for sure haven’t seen yet or else you’d be right there with me. Just look at this website and…”
There’s no form of productive engagement with a conspiracy theorist. Much like an Internet troll, engagement of any kind validates their effort.
Yeah, pretty much this.
“While we’re on this topic, did you know that Lincoln was shot by Robert E. Lee in disguise? Booth was just an unfortunate patsy.”
Oh, I don’t know. I got cornered by a conspiracy loon at a family event one time who knew I worked in technology and wanted to talk about the way cellular-signal radiation cooks your brain when you hold the phone to your ear. I handled it by snickering, and asking leading, trolling questions: “So (giggle) you think the 3G signal [this was a while ago] heats up the cerebral (chuckle) the cerebral cortex? Really? How does that work (big shit-eating grin)?”
These people want you to argue with them. They don’t want you to laugh at them.
He got frustrated after a couple of minutes and left me alone the rest of the party.
Too in-your-face/hostile. I was going to suggest walking away while visibly trying (but failing) to not laugh out loud. This approach protects you from accusations of rudeness; afterall, you did your damndest to stifle your laughter (even though there was only so much you could do).
My daughter came home from nursery school one day in tears because Martin Luther King had been shot, and he was a good man. Somehow, pointing out that it was a long time ago and he’d likely have been dead of old age by then did not comfort her. 
But that’s my real answer. They’re all dead, so it doesn’t seem very important.
This also works on Christians.
“Jesus? Sorry man, I’m not from around here. Don’t know the dude.”
Or, you could just nod knowingly and then add.
“I agree. Also, birds aren’t real.”
Just change the subject.
“So, whaddaya think about 9/11? Chemtrails? Vaccines?”
“But I repeat myself.”
Oh yeah go all in, the lizard people cabal,
And how they’ve infiltrated the Illuminati and control space lasers. Then Shrug.
“Sorry but I don’t want to know the truth. If I knew what you know, I’d have to worry about how secret government agencies would begin watching me.”