Kayla and Rob are good friends, and they also happen to be taking a class together at a community college. The class is very small, so the professor knows all of the students by name. One day, Rob decides to play hookie. He texts Kayla and tells her that he "got really sick and won't be in class today." Kayla writes back an acknowledgement and a get-well-soon, and goes to class. The professor notices that Rob is not present, and casually asks the class if they have seen Rob. Kayla pipes up and says "He's sick today."
Eventually, the professor finds out that Rob was not actually sick, and just took the day off. He also happens to know that they are friends, so he is disappointed not only in Rob, but also in Kayla for "lying" to him about Rob.
Kayla did not tell the truth, but pragmatically she did not "lie." She merely forwarded false information that she believed to be true, as she was given that information in good faith by someone she trusted.
I'm looking for the most succinct, accurate phrase/expression/word to describe "giving someone false information while believing (and having good reason to believe) at that time that the information is true."