Echoing what everyone else has said above. Emotional dishonesty is not a foundation for anything good -- and that is what a relationship with a married man would be based upon.

If his marriage sux, he needs to cure it or kill it. In my experience, it takes all the emotional energy two people have in them just to keep a marriage solid and functioning. Siphoning a bunch of that off towards a third person is chaos waiting to happen, and you say you want his marriage to work out and for good things to happpen for him. Listen to your morals: You can wish good things for him, but you cannot BE those good things for him. He's taken.
That said, it's not a sin to be attracted to someone: it's one of the ways we learn what qualities attract and inspire us, hence what is important (or less so) to us in a potential mate.