The operative principle here is "three can keep a secret if two of them are dead". A secret can only stay secret if everyone who knows it is unanimous in the desire for keeping it.

Alice can keep her own secrets. If Alice sends a secret to Bob, she is now dependent on Bob's willingness to keep it. But if Bob wants to share Alice's secrets with Eve, there is no scheme possible that would allow Alice to tell Bob and not Eve.

The best Alice can possibly do is identify Bob as the person who told Eve the secret, and then visit him in the dead of night and beat the willingness out of him with a PEX tube full of sand.

And if Alice wants to do that, she has to use a watermarking scheme that is undetectable to both Bob and Eve, otherwise Bob can remove it before sending the secret on to Eve, or Eve can strip it out as she gets it from Bob. You see the problem, here, since the whole reason Alice is doing this is because she has some difficulty keeping secrets. She is essentially hiding a steganographic message to herself inside her message to Bob, such that if she can detect it in any message from Eve to anyone else, she knows to pay Bob an angry visit.

Alice's real problem is that she's an abusive, crazy, dramatic, jealous stalker, to the point where Bob would usually rather deal with the shady, disreputable Eve than with her. As a result, Alice's schemes are often the figurative equivalent of "Bob, unlock your phone so I can read your mail, texts, and tweets," and sometimes even literally that exact thing.