Would she stand up for bullies in school, because picking on the weak strengthens the social hierarchy? How does she feel about aristocrats keeping dwarfs around to give them a sense of superiority? What about beating children or small animals to keep them in their place?

People can feel powerful in positive ways: They can play games, exercise, do a good job in their field... They can defend and protect the the people they care about!

If he's feeling powerful by harming others, particularly the one with whom he's in a relationship, his fleeting sense of power is improved, but the relationship isn't because one half walks away with injuries. The boyfriend alone is not the relationship.

But I'm preaching to the choir here. You and I know that, and so does your boyfriend.

I'll bet that reasoning was a hastily thought up excuse by someone in denial. She knows her logic is flawed. You know, some people won't take anything seriously unless they hear it from a professional, like a doctor, or police officer, or someone else with some respectable title.

I would suggest reaching out to her. What I have found to be effective with several people close to me is to offer a completely nonjudgmental ear. Listen, reflect back, ask challenging questions, and although it will be very hard, do not say bad things about the boyfriend nor tell her that she should leave him. What you need to do is help her to come to that conclusion on her own. Ask questions like "What do you want out of a relationship?" Ask her questions and reflect back to her statements that will bring her to the conclusion that she is not happy and fulfilled in this relationship. It will take time, but is has worked for me several times so far. In addition, please keep in mind that leaving a violent relationship is often more dangerous than staying in it. Therefore, when she does decide to leave, please help her set up a plan to go to a shelter so that she is safe.

As painful as it is to watch, you can't help her until she realizes that she needs help. Let her know that whenever she needs to talk, you are there for her. And don't let Y know that you are in contact or he will forbid it! Give her your number in private, if possible, and tell her you'd like to be friends and she can call you to chit chat or hang out anytime. Don't make it about her relationship or she may feel pressured. Just make it about friendship and a safe space.
If she takes you up on it, offer a non-judgmental ear, and don't criticize her rationales or you may drive her away. Definitely ask her questions to draw her out, but try to avoid judgmental wording. ie, try not to ask things like "But isn't it better if he doesn't need a sense of power?" try phrasing like "do you like that power dynamic in a relationship?" without any implied correct answer. When she starts to feel more secure with you and sees you as a friend, you will be on better ground to offer opinions and advice. She will be ready for it on her own schedule.

I'm thinking sneaking into X's house and something to do with something sharp. However, thats just me....