It's sad to see so much judgment being passed on this situation. I agree with other people who said that these kinds of situations happen everywhere and in all walks of life. One spouse leaves another for the co-worker, her dentist, his neighbour, the best friend, the teacher, the student, the boss, the patient and the list goes on. This one just happened to be in the dance world.
Who are we to judge How, When and Where people fall in love? Not all relationships are meant to last forever. It's a fact of life, not something that should be viewed as shameful, outrageous or whatever you want to call it. Sure, people get hurt because we're humans: he have emotions. But given the choice between staying in a relationship that doesn't work anymore and finding happiness with a new person - which would you choose? (assuming you're not a masochist)
Also, why point the finger of blame at the person who instigated the alleged breakup? BOTH people are responsible for their actions. The person who leaves their spouse is equally responsible because they made that choice consciously. They weren't drugged and forced to make that decision at gunpoint. And if those choices lead to a happier life, who are we to get all moralistic about it? It's their life! And their business, not yours! How would you feel if everybody around you criticized YOU for the choices you made in life (regardless of how big or small)? If you're feeling outraged by it all then you need to ask yourself why it's triggering that kind of response in YOU. You will most likely find that it's pushing one of your sensitive buttons in which case, that's an issue you need to address in your own life vs project it onto someone you barely know.
Get off your high horses and get a life. Focus on bettering your own life vs judging other people and their choices. Instead, you should aim to respect other people's choices and hope that it works out for the best for all involved because in retrospect everything happens for a reason.