People tell me I’m too nice, but they never turn away when I am nice to them
If you don't feel the same way about him, if you're just leading him on, you need to tell him that. I've seen too many nice guys get shafted because a girl can't get over some jerk.
Perhaps your desire will help her see what you see, but if she cheats on him with you, there’s a good chance she won’t leave him. And you’re left where she is with a jerk. Take it from a nice guy and a wallflower.