If you haven't guessed by now, the reason he doesn't like Sisko and by extension DS9 is because of how Sisko treated Picard in their first meeting on board the Enterprise in Emissary. I should have thought of that because my friend is very much a part of Team Picard
Let me get this straight. Your friend was extremely attached to someone. That person, Picard, was attacked by Sisko's actions. On a single occasion, years and years ago. Under circumstances which have long since been justified to some extent.
And yet your friend won't let it go after all this time, choosing to continue disliking Sisko regardless.
The irony is appalling.
It is not linear.