Is there any possibility for you to be what he claims he is trying to be for her? I don't mean sexually but the friendly support she needs but can't utilize when it is him for the romantic angle.
I experienced this dynamic early in my own relationship. I'd met someone fresh from a break up with a troubled ex partner. He still cared for her and wanted to be a supportive friend for her struggles but it was the very fact that they had been involved that she couldn't take his support and not want to be in a relationship with him. So I tried to be there for her myself.
In the end it didn't work. She couldn't let me help her either. She acted out in one last desperate move to be with him. It made him realize the only way he could help her was to make her stand on her own and trying to help any other way was keeping her unstable. He saw that if he couldn't fix it and I couldn't fix it there was just no room for her if he wanted a life with me.
But had she been able to see me as a real friend, we could have learned to care for each other and who knows where that would have led?
Do you have it in you to give that a genuine shot? Because if you cannot there is nothing in it for you to relent your opposition to this woman as a possible other to him.