I don't know of any commonality of philosophical response to the predicament you describe. It is hard to see how the mere state of Daniel's being in love with Rose could be 'sinful' since it is (presumably) involuntary : I at least can't recall deciding to be or remain in love with someone.
Nor is there, as you describe the situation, anything sinful or as I prefer to say morally wrong in the mere texting that occurs between Daniel and Rose. You don't specify the content of the relevant texts.
Where if anywhere philosophy comes into the picture, if we are to raise responses to the question above the level of mere personal opinion, is in the significance of personal autonomy here.
Rose appeals to her autonomy in her relationship with David. If she decides on the basis of deliberation to follow certain rules or standards in her life, if she can voluntarily act on that deliberation, and if one of her rules is to compartmentalise her various relationships, then it is hard to see what critique Daniel can apply to Rose's decision not to let David know about the texts.
By parity of reasoning, however, Daniel can appeal to his own autonomy. If he decides on the basis of deliberation to follow certain rules or standards in his life, if he can voluntarily act on that deliberation, and if one of his rules is to tell the truth whenever he decides it is important to do so, then his ethical relationship with Rose is plain. He (a) respects her autonomy, which extends only to what she does or doesn't do, if he does nothing to control or coerce her behaviour, and (c) he preserves his own autonomy by telling David about the texts if he cannot persuade Rose to open up to David.
However, to take matters down to a fully philosophical level one needs to anchor this answer in a particular theory or account of autonomy. There is a diversity of such theories. You should perhaps make a start with Kant (Foundations of the Metaphysics of Morals, 1785, is a key text) and work up to current work by Christine Korsgaard, Thomas Hill, Harry Frankfurt, Gerald Dworkin, Wright Neely - this is a list, not a complete enumeration. The internet provides an introduction to their views.
