While I agree that as an adult, the decision to end the toxic relationship with his GF is his decision, having made my way out of an abusive marriage, I know concerned family members and friends don't need to just sit there and wait. There are things that helped me while I was still in the thick of it.
1. Remind him of his great qualities. She's beating him down. When I'd get to see my mom or dad and younger sister, they'd all point out what a great momma I was / am to DS. What a great aunt I was / am to my nephews. My mom would reminisce about how amazing I did in school. "Do you remember, TU, when you made the Chancellor's list while still holding a job? And how often you did that? Chancellor's List is getting 4.0's, right?" My dad would remind me of how strong and courageous I am, of when I stepped between a high school friend and her abusive stepfather when he started in on her while I was there (and then of how glad he was that the guy dropped his arm as I stared him down).
2. Remind him that he is loved for who he is. She's imposing her vision of who he is on him. If she's like AXH is, she's probably convincing him, in very subtle or not-so-subtle ways, that she is the only one who could put up with some one as flawed as him. Continue to show your love for him.
3. Listen to him, to his opinions. All that matters in his toxic relationship is her opinions. AXH would constantly tell me that I was stupid. I wanted to be a writer and he'd scoff that no one would want to read what I had to say. My family would discuss current events and if I was quiet, they'd ask what I thought, I'd often start with "AXH thinks..." They'd listen but then ask, "That's what he thinks, what do YOU think?"
4. Encourage him to do stuff outside the relationship. I didn't get to do much besides take DS to my Dad for daycare duty and go to work and be at home with AXH. My sister would show up with her kids in tow and take DS and I to the museum or sledding. They'd tell me about events coming up at the Museum or park strips. They'd ask if I'd reconsidered the writing program at the university or law school.
5. Don't shame, blame or guilt him. He gets that enough from her. I got it from AXH in heavy doses. Don't tell him "I don't know why you stay with her. You just need to leave." Let him know you understand he loves her, but you're concerned and ask how you can help him. Focus on facts. Don't be surprised if he gets defensive and don't respond with anger or put your own defenses up. I defended AXH over and over and over. My sister simply stated what she'd seen or heard and that she was concerned for my happiness and well-being. I may not have and probably didn't respond well to it, but I remembered.
Remember, just as the toxic relationship affects him, it can also affect you. If you feel you need help dealing with it, please remember that you have a right to your own support system, too.