Remember that love is free. It transcends space and time. It does not require physical proximity. Love is an ever-flowing energy and yet it is also tangible and intangible, imaginative and real, particles and waves. Love is an infinite paradox because it is all things.
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.