هُوَ لَكَ مُصَدِّقٌ وَأَنْتَ بِهِ كَاذِبٌ depicts two simultaneous conditions: the listener believes, while the speaker is lying. That contrast highlights the betrayal.
In the first nominal clause, هُوَ is the subject and مُصَدِّقٌ the predicate. In the second, أَنْتَ is the subject and كَاذِبٌ the predicate. بِهِ refers to the reported matter.
When unsure, saying “I have not verified this” is better than pretending certainty and abusing someone's confidence.