Quoting Gobble, gobble!:" To love your DH while being attracted and thinking about someone else 24/7. What would you do? Leave ... [snip!] ... if the guy lived in the same neighborhood as you. Two houses down kind of same neighborhood? Sorry for any mispelling lol."
I'd say she needs to evaluate her relationship. There is such a thing as "falling out of love" where you stop putting effort in. It's VERY easy to think things would be so much better with someone else becaues you see the worse of your husband, but only the best of this other person. The grass isn't always greener on the other side, it's greenest where you water it.
If she truly feels she is DONE with the marriage, then that is different. It has NOTHING to do with this other person. IMO you should never leave a relationship FOR someone else. If you leave a relationship it should be because it's best for you.