It's because he doesn't have respect for women. It's time for his girlfriend to run like the wind because respect is the foundation of any long lasting relationship and strong* guys don't have this type of behavior.
*A strong person has good character (honesty, integrity, trustworthiness), displays a positive attitude (cheerful, caring, friendly, forgiving, helpful, and respectful), fulfills their responsibilities (for handling people who are pains in a positive way, for always trying to make a good choice, for taking care of themselves, for serving others), gives their best effort, and demonstrates self-control (of their body, anger, mouth and money).
Hope this helps!
True Love Lasts - written with a character emphasis for teens through young adults,
Straight Talk About Teen Dating - written with a Christian emphasis for ages 13-19, Straight Talk About Dating - written with a Christian emphasis for ages 20 and up