Ginny was grateful when Neville interjected because honestly, what was coming next was not something that one said to one's mother. Especially when one's mother was Molly Weasley. His taking the attention off of her bought her enough time to collect her thoughts and calm down, even if it was just marginally.
That momentary calm was short lived, however. Neville's suggestion that they get a place together caught her completely by surprise. She gave him a look that was a strange mix between surprise, confusion, panic and excitement. "Um...what?" This was not exactly her idea of the right moment to be discussing this for the first time.
"Excuse me?" Molly said, clearly equally shocked by this sudden development. Though she was a bit more horrified looking than her daughter.