The scene in which Ravus tells Luna to stop puting her life at risk happens in Tenebrae (and it has to happen after Chapter 5 (Ravus already has magitek arm and Luna has already waken up Titan and Ramuh) and before Chapter 9, both Ravus and Luna are already in Altissia).
You learn that Ravus has been seen crying at Yureil Plaza by optional dialogue. You actually get to see the follow-up scene in which Ardyn approaches him (I'm not sure it's missable, but here it is:
https://www.youtube.com/watch?v=C6TPpjDeSO0)
Then, after learning about Luna's impending fate, Ravus meets Luna before the Covenant begins. And through chapter 10 to 12, you keep learning about the Empire blaming the Altissia incident on Ravus and sentencing him to death.
It doesn't help that this game's story sounds so much better when you write it than when you actually play it.