not sure but the gunslinger better find himself in a duel again. Maybe the book lets them know that the will of his god is to not shame him like that again.
That sort of stuff is deeply frowned upon in dueling circles, so much so that most (non-old western) duels would have each participant bring along a “second”, someone who was obligated to kill those who interfered or duelists who cheated. It’s likely that any friends and family of the deceased would be feeling pretty raw and might be out for revenge. Even some higher-ups in society or dueling commissions could be looking for him, just so they can make an example of anyone who breaks the rules.
As far as the book goes, you could try elaborate rituals or sacrifices to get it to work. Maybe go on fetch quest to get items or materials that are important to any given god. From there, maybe dark prophecies about rising evils or dark perils facing the PC’s in the future. Either way, I’d recommend having them show it to a priest first.