How do i win her heart?

You can't change what she thinks. You may just to too special to her as a friend. However, telling her that she is better than this guy will help her see that she can do better, and by better i mean you. Be the shoulder to cry on, and if they break up, then you can think about moving it forward. By always being there for her, she will like you more and more even if it doesn't feel like your getting together.