You can definitely have an honest conversation with him about how you're feeling sidelined as his friend. Try not to blame the gf as he might get defensive, but focus on your feelings about the friendship. When people are in toxic relationships unfortunately their toxic partner will try to isolate them and cut off their friends. So the best thing you can do is try to be supportive, bolster his self-esteem, and make it known that you are there for him especially if he decides to leave her. That said, you never have to subject yourself to bad treatment so if it does go in that direction make sure to have boundaries to protect yourself.