I don't react, she doesn't get the satisfaction of drama or closure from me after something like that. If she told me or I found out I'd just say 'we're done' and leave. Her calls/texts would go unanswered, I wouldn't answer my flat door to her, if I saw her in the street I'd act like I didn't even know her. She'd be more than dead to me, she'd be as good as nonexistent in my eyes. If you know anything about girls you'll realise that's the worst punishment you can inflict on a girl who has hurt you. No shouting, throwing, crying, no emotion whatsoever. It'd drive her insane.