Today seems to be the day for ethical dilemmas. Some good ones here, too, good enough to probably be worth collecting as case studies for an ethics class.
If this were my friend, I think I would be as much concerned about the wrongdoing of someone close to me as I was about warning her before she repeated her theft when the stakes were high. If this were my friend, I believe that I would bookmark the published drawing and leave the book in her path. If she stole it from that book, she will know it by its cover without ever opening it. I would expect her face to tell me volumes.
What I would not do is fake or pretend anything while talking to her. This would be a time for honesty above all else.
I would want her to know that I knew what she had done, and I would want her to understand that it could have serious consequences for her, not just now but in the long term. Scenarios are easy to imagine. Suppose she makes a name for herself, and her buyer brings out this example of early work—and it is seen by someone who knows the original artist’s work? What would that do to her career?
I would also realize that I might be losing a friend.
Any follow-through would be up to her. She should contact the buyer, buy the work back, and destroy it, but I would not tell her that. I would just quietly let her know that representing another’s work as your own is an act that has a name and that can have serious consequences. I would not be the policeman to contact the buyer or the owner of the original or anyone else. That is her responsibility alone.