If a woman is married, but it’s not a very happy marriage, and after a few years, there is another person that enters this person’s life who seems to be a better match, can Istikhaarah be made regarding this other person?
To have any casual contact with a non- Mahram is Haraam. It is totally wrong for a woman to entertain thoughts of any non- Mahram while she is married. To make Istikhaarah in such a situation is totally misplaced. She should work towards improving her marriage.