She didn't know what to say. How could he possibly know what she'd said to Aberforth in a private conversation? She knew there was no one around the Shrieking Shack who could hear them. Maybe it had been wrong to lie to Aberforth and then to Charlie.  She didn't care. They needed to know that she couldn't be trusted. Sure, she could've easily told them that it was actually Rodolphus who claimed that Aberforth was aware of his presence in the village. If she did, however, they might have continued to go on trusting Ginny when she was dangerous. The crumbling Resistance had a rat amongst them that they needed to know about. At the time she didn't know that Ginny only did what she was doing because Rodolphus was forcing her, but what was done was done. There wasn't any way that she could take it back. Ginny had been exposed.