2) If the listener is not allowed to believe the informer that he [the informer] heard it in front of three, he should not be allowed to listen. By extension, the informer should not be allowed to tell him, because of לפני עור. So to say that it is muttar to repeat a story heard in front of three but it is assur for one who hears it from him to repeat it is paradoxical.
It’s not lifnei iver, because although the listener has to be choshesh that it wasn’t actually said b’apei t’lasa, the informer knows that it in fact was, and that the listener was not actually oiver on anything.