Girls can be very bitchy

This girl has a boyfriend well she won't let this guy do anything unless she approves it but she never wants to meet up with him ever and they've been "together" for 3 years. She thinks I'm going to get with him but honestly I'm not interested but she made him unfriend me. 
He says we are still friends but he doesn't talk to me anymore because of her. 
How can someone be so controlling and never want to meet up with their "boyfriend"