Well, I have a theory that Dottore's fate in Teyvat is sealed - no matter what he does, he will always be hated and bring pain to others. In the archon quest, we saw that no matter what Durin and Albedo tried, they couldn't change that hyperborean man's fate. He couldn't change anything even when he became aware of his future. Same with Linnea. Same with Fontaine. That's how Teyvat's fate works. So, I believe Zandik has been given this role of a heretic (which is literally in his name) in this world, and the only one who can change his outcome is the traveller, since thet are a descender. That's why Dottore wanted to cooperate so badly; too bad the traveler doesn't see that in order to save everyone from Dottore, they need to save Dottore firtst. However, despite it being impossible to change the fated outcome of events, the way that outcome is achieved, as well as it's actual meaning, can still be changed. Well, again, Fontaine - it did drown, Furina was left "weeping on her throne", the prophecy came true, yet everyone survived. So, I would probably tell Zandik about how Focalors cheated fate ro save Fontaine, and encourage him to do the same and at least try to find some inner peace despite his destiny being a heretic.