No does mean "no," as in, "No, of course I'm not going back to your apartment with you and your weird, drunk friend. You think I'm an idiot or something?"
If only this girl's parents had taught her some common sense.
Well, exactly. "No" in that context is clear and unambiguous. That being said, from what few facts we have here, it seems likely that the pair are, in fact, guilty, but we'll have to wait and see what happens.