The quest log just says convince Regis with evidence. I have the diary and tried looking for a dialog option with him to use it but no dice. I outed the advisor with speech checks so I didn't need to use the diary. Tried talking to Papa and he just says some of his advisors have doubts about the alliance but that is all.
Ok, that was probably the issue. This quest has a bug where the Speech check doesn't actually help. From the Wiki:
If you use speech to discredit Karl or convince Papa once the four Khans agree to break away from Caesar, you will be unable to complete the mission, and For The Republic II, it will say you've killed all the Great Khans.
We're supposed to be getting a patch soon, maybe this quest is among those fixes.