It took Severus a few moments to figure out the mumbling and then it took another few moments for his brain to get around the fact that Lily had expected him to invite her. He would do that in a heartbeat but the fact that it seemed that she wanted to go to a ball with him, was a big surprise.
"I don't get invited to fancy things like a ball." He stated trying not to sound to bitter, but then he got an idea. He looked around to see if they really were alone, when he didn't spot anybody he got up and extended his hand to Lily. "Why don't you show me how to dance at a ball." He hoped it would make his friend smile again.