First of all I'm sorry you're going through all of this! Second of all, to answer your question at the end of your post about if their marriage is going to last, my answer is no and I'll tell you why. Because what they are experiencing is not love, it's limerence. Limerence is basically an "emotional high" that people feel that they confuse for love. It's that "love at first sight" and "you're my soulmate" b/c everything they could ever want in a person is all wrapped up in this "perfect" individual. It's a chemical in the brain that basically makes a person irrational, and if it happens in a marriage, then the spouse that is being hurt by it will feel hopeless!
The good news is, it only lasts 6-36 months. That's how long it takes for it to run it's course. At the end of it, your ex-husband may come back to you and say how wrong he was about "her." Not saying that will happen, just saying it might.
The reason I think it is limerence is b/c of how you described his actions...his "true love", he says he had pity for you and never really loved you, she is giving him "what he needs" that he isn't/wasn't getting in your marriage.
All those are pretty good signs that this is temporary. When someone starts saying "I never really loved you" that is a HUGE sign that they are infatuated with the other person and are changing history to make themselves feel better about leaving you.
So all that to say, eventually it will end. And he MAY come back to you and beg forgiveness. A great book for you to read is "Your LovePath" by Joe Beam. It explains limerence in more detail and also explains the love path. It may help you in understanding what happened in your marriage and give you insight on future relationships. You can find it at http://www.joebeam.com/books.htm
Hope this helps!