How to destroy a shaky relationship?

I moved in with that chick because she apparently really wanted to live with me. (I'm not sure why, it seemed very important to her, to the point of kicking out her previous flatmates.) Now she has a boyfriend who's an asshole. Man, what a douchebag. He doesn't have the slightest bit of respect for her. How do I destroy their relationship?

She doesn't want to fuck you, just to make you jealous and to fuck with her botfriends head. Typical woman.