Okay, this actually gave some insight on her constant abuse: I think it's her defense mechanism.
She generally puts him down whenever he tries to do something to impress/compliment her and when he's down she mockingly cheers him up just enough to get him out of the depression abyss but still low enough to amuse herself, but this one time it was different: she was genuinely taken back by his sudden "coolness", I dare say she even panicked, quickly rationalized where he got the ring and verbally abused him even more brutally than usual.
I think she might be so terrified of being in a relationship with someone on equal standing that she physically cannot stop being abusive towards the poor guy.