Advice for defense against job!

Hello Sir, My brother, 45 years old, married and has one child. He had been messaging my neighbour's daughter(19 years old) for 2 years through facebook. The messages were flirtatious but were not threatening. The girl read all those message but didn't reply and neither blocked him . After two years, we suddenly came to know about all this messages from that neighbour. They were furious over our family.According to them their daughter didn't like those messages but stayed quiet for two years. Their family came to know about this well before the completion of this two years but never informed us or blocked him.Between this time my brother was invited to their house for various occasion several times. And I'm pretty sure they invited him even after knowing about all those messages. However, my brother apologised and stopped sending any further messages. Now they have filed a complaint against my brother at his office to the chief engineer after two months. Now, his job is at risk. Now, can you tell me what to do? Should we fight to defend his job?