Having a crush and a fantasy life is one thing.
Acting on it is another.
I don't know how your husband's friend would react if you came on to him. And what if he outright rejects you and you make a fool out of yourself?
And finally why would you want to break up a friendship between your husband and his friend?
If you're this miserable in your marriage, maybe you ought to focus on why you're unhappy with your husband, and, what your husband can do to ignite the spark. If not, you married the wrong man, or you married for all the wrong reasons, or you're confused.