Ethan is eventually going to come back of his own free will at the very end of what I think of as book one, and at that point he will explain everything to Bree and they will find a more official way for them to work together. But right now he is still convinced that the thing he truly wants is to just get away. Bree's going to need to, one way or another, drag him back for this one mission. Only this one, I promise.

What if she didn't think of it as guilting? What if she just thought of it as logical reasoning? After all, Ethan killed the only lead she had, so, since he can aid her, he should be the one to replace Victor until the case is over. Would that work?