In every edition of D&D that I'm aware of, demons seek to spread disorder and despair. Therefore, the terms should seek bring about that state of affairs. Ideally, the outcome should be one that makes the player question whether this deal was worth it. In particular, a deal that seems understandable and barely worth it at the moment it's made becomes one in which hidden costs are revealed over time, making the value of the deal more and more questionable.
The obvious way to achieve this is one where her ability to speak about this topic causes an unwelcome turn of events. In other words, the situation would have turned out better if she'd been compelled to maintain her silence in the first place. Therefore, the demon benefits twice: once from whatever she promised in her pact, and again because the "benefit" she got only made things worse.
There may be other ways to go about this, but it seems to me that there are two general paths to follow, assuming your demons are like mine. The first is where the character's actions cause disorder and despair to the environment. In this case, the cost depends how guilty the character feels about her actions, but doesn't necessarily impact the character herself. This could be a path to the eventual corruption and "fall from grace" of the character. The other path is one where the personal chaos and unhappiness of the character is increased, which is more direct and obvious in its costs.
One overall consideration is whether this is the "correct" or "incorrect" choice on the player's part. If it is "correct," the player should be rewarded (in terms of interest or benefit) regardless of what happens to the character. If it is "incorrect," it's the player who should regret their choice. However, in either case, you should bear in mind that you do not necessarily want to inflict despair on your own player. This, as in all in-game activities from my point of view, is to increase the engagement or overall enjoyment of the game as a whole. I would hope that the point of the exercise isn't to lose the player.