Girls are so violent these days..
Its like this, Shes just trying to push your buttons to piss you off. FORGET about it, dont let stuff like this bug you. I know its not right, but if you get pissed and play her games its only going to make things worse. If she realizes that your not affected by it then eventually she'll lose interest. Becuase whats going to happen is if you play her games shes going to work harder to get him away from you, whether it be cheating on you or what.
This is where he has to take a stand also and be a big enough man about things to say "this isnt right!"