I think Horan will attempt to overwhelm you with the facts of the case, pointing out unexplained events in the case narrative that his theories attempt to account for. Conspiracy theories tend to follow this pattern (i.e., the conspiracy theorist fills every gap with his theory). Assuming his theories don't hold up, then your job will be to show that his explanation is not the only possible one.
If you argue with him on his level, you will be forced to present and justify alternate explanations for all the "holes" in the Zodiac narrative. This is how I end up approaching most claims of solutions to the ciphers: Generally speaking, the would-be solver has presented a solution but ignores the many others that also reasonably fit using the same approach. Horan's theory can probably also be categorized this way, since there may be multiple rational explanations for each piece of evidence he highlights. But an idea about the Zodiac case is only truly useful if it is shown to be the *only* rational explanation (i.e., the Hardens' key to the 408 is the only rational solution.) Otherwise, the idea can only be as good as all the others. If all Horan can do is show that his idea is as good as any of the others, then he hasn't proven anything.
This is a scientific approach. An idea only holds weight if it survives the tests. But even so, many people don't care about that. I once listened to a debate between astronomer Phil Plait and entertainer Joe Rogan. They debated over whether or not the moon landing was hoaxed. Rogan presented all the supposed "gaps" in the narrative and physical evidence on the moon landing, and Plait attempted to address them all. While Plait had the force of reality and evidence on his side, Rogan was much more aggressive and effective at pushing his message across because of his long experience as a broadcaster and stand up comedian. He also injected a lot of emotion into the topic, which listeners latch onto very readily. Be careful to not get too emotional when engaging with Horan. But then again, maybe the right kind of emotion makes listeners change their minds.
It's going to be tedious work and I don't envy you for it.
