Most likely her partner feels/knows, on some level, that she is better than him and could probably find someone else in a heartbeat. Her partner probably has very low self-esteem himself and doesn’t want to lose her. Even knowing that he does things the wrong way, he won’t stop himself, because he thinks that with her low self-esteem, he can lash out at her, while knowing that she won’t leave because she loves him. It’s definitely a control game he plays, just to keep her in his life. If she ever decided to get up and walk away, he would probably show an unbelievable amount of love and kindness – so much that it would either match or even outdo the amount of abuse he’s shown, which would only verify her feelings of being in love with him and thinking that he actually loves her.
He might, on some level, actually love her in his own twisted way. But individuals like that are so screwed up that they have no idea what it means to truly love someone, which is why they believe in the first place that they have to manipulate people to keep them in their lives.