1) Thank them for their honesty. 2) Walk away. Somehow the moment they stepped out of the relationship it was the biggest sign that it was over. No matter what we do to work towards the future the past will always be and knowing they could be so selfish and be with someone else well... It would kill me. *Honesty+Loyalty are the most crucial elements and when either is missing then why even be in that relationship? With so many humans that exist, why give to someone who disregarded your worth in the arms of another?