My guess is that this all came out in deposition and the defense WANTED her on the stand so much that they would have called her if the prosecution had not.
Now that would make some sense. If the prosecution knew the defense was going to call her they may have decided they were better off calling her themselves to frame the situation and limit the damage as much as possible. But if that was the thinking they would dealt with the letter during their direct examination and tried to explain it away before the defense got its turn. I'm not sure that happened here.