Ron clearly hadn't thought his plan through, as Hermione stormed in. Because now there was no hiding the fact that he had been drinking from her.
But instead of answering her question right away, he turned to Harry and said, "See? Works really well," before switching his attention to Hermione. "We're drinking? Didn't think it would be a big deal, since..."
He shrugged, definitely not wanting to finish that sentence.