As others have said, is the secret relevant to plot or to character development? Depending on which it focuses, there will be different timing to it, usually. Secondly, how big is this secret? Is it enough to change C1's opinons (and possibly the reader's) on C2, or big enough to maybe change even our perspective on the plot as a whole?
What I want to get at is: Does the secret make you re-evaluate all of C2's actions and plot involvement so far, significantly changing how the plot and C2 will be perceived moving forward? I ask because, usually, conventional story structure suggests these big perception-changing revelations be at the Midpoint of the story, so as to give you enough previous material to re-evaluate, while at the same time enough material going forward to explore the new circumstances. Too much of one over the other can be tiresome or loose impact. By the Midpoint the reader is emotionally invested enough for the revelation to mean something, so that's the ripest time to do it, conventionally. If you wait too long, the reader feels cheated, as important information that would colour the whole story differently is withheld (depending on the nature of the secret, you could use it as a twist, which lets you stretch it more). If you do it too soon, the reader won't care.
Then again, it depends on the nature of the secret and how significant it is to the characters' perceptions and storylines. If the secret is more on a personal note, mostly driving character development, then you should consider C2's character arc, as these arcs intersect and run in parallel to the plot, but not always at the same pace. What might be the right timing from a character development standpoint, might not be the same from a plot standpoint. An arc/subplot has its own structure within the main story framework. I can't say more without more information about the story.
But anyway, if all other considerations fail, just boil it down to: when can I squeeze the most conflict out of this revelation? Either to finish a segment of conflict in the story, or to start it, or to extend it, or to intensify it. You can almost never go wrong in this direction.