Is it permissible for a girl to perform Istikhara regarding marriage to a religious and well-mannered man who is already married?

It is permissible for a girl to ask Allah to marry her to a specific person, even if he is married, as long as he is a man of good character and religious commitment. However, seeking Allah's guidance (istikharah) for marriage to this man is meaningless, because istikharah is performed when one is contemplating a matter, and the man has not yet proposed to her.