My friend is dating a guy she used to totally disrespect. What's wrong with her?

She's dating this short little nerdy guy who spends all his time talking about bankruptcies and mathematical modeling. She used to make fun of him, call him a nerd and brag about how she was using him all the time. She kept saying they were "just friends" and he knew it. She even told me she had "nothing romantic" going on in her personal life. However, she had been hooking up with this guy, letting him take her to dinner and going on trips with him. The whole time she had been hitting on other guys including me. The guy turned out to have major little man's complex. Is rude to people, snubs people, is rude and even threatened a mutual friend with violence. I'm shocked she hasn't dumped him. What's wrong with her?