I love your sympathy.
Not sympathy, silly. Not often is it I feel sorry for people. Empathy is the word here. I understand the effects of building a relationship on lies and deceit, and I've seen it all come out in one quick lash. I also understand that such relationships, once collapsed, can be rebuilt. You just have to learn to find a new foundation, as a house built upon sand often falls quickly.
Then again, this incident may be a sign that they shouldn't be together. Perhaps they have or will part their separate ways. But still, that's no reason to leave the relationship in complete disrepair. Of course, they may never have such a relationship at the level they did before, but this does not mean they should have a negative one.