How can girls be so disrespectful

How can girls be so disrespectful and go for a guy that already has a girlfriend. There's this one girl who knew my boyfriend for about three years. My boyfriend and I started dating in August of last year. And this girl got into a relationship in January. But all of the sudden. She keeps calling and calling my boyfriend. Bothering him. And recently I found out my boyfriend was cheating on me w her. I found some Facebook messages between him and her. It started when she broke up w his boy. And damn. Pathetic ass girl. She had the nerve yesterday to hit up my boyfriend crying telling him that why is he preferring me over her. And she called me a fatass and that she is way prettier. All this bullshit. And I told my boyfriend block her in every social media you have. Well that asshole blocked her. But the next day she sends him a pic that said that she is over here stressing over him when he doesn't give a fuck. And I tell him I thought u blocked her. And we got into an arugment.