Granted I'm not a Titans fan, so obviously I'm missing a lot of the context here, but why exactly would Cassie feel the need to apologize for something that happened while her boyfriend was dead and their relationship - for all intents and purposes - was over?
If Tim and Cassie had carried on a relationship or were still in one at the present an explanation (rather than an apology) might be in order, but the way it's framed definitely seems to suggest that there really weren't a whole lot of romantic aspirations tied to the incident to begin with.