There's a lot of loaded language in this post. I'm interested to hear how it is considered "backstabbing" to ensure you get everything you are legally entitled to?
Well, manipulating someone to make him think you still love him when you don't, keep him in the dark about a massive change going to happen in his life, and then hit him hard with it and get away... Are you really going to pretend you don't see how it can be considered backstabbing ?
In fact, you recognize yourself that you're going to devastate someone emotionnally, and that you're as such trying to do it from the shadows. That's not just "considered as", that's the very definition of backstabbing.
You simply cannot trust people to act in a civilized and decent manner when you are about to do something to them that is going to devastate them emotionally. So, as terrible as it may sound, you really do have to get them before they get you if you want any chance of getting what is rightfully yours.
That's a truly horrible way to look at mariage - and at human relationships in general. That's how you wage a war against a dangerous foe, not how you manage a change in your love life.
Unless your soon-to-be-ex-partner was truly a jerk, there is no justification to behave in such a underhanded way. In fact (unless, again, s/he's been a true jerk), you OWE her/him decency and sympathy. Years of relationship don't count for nothing.
Plus, people don't just up and decide they want to divorce their spouse for no reason. Although I don't know the situation, my observation of friends and family around me that have been divorced leads me to believe the husband in this situation isn't an innocent victim. He has obviously done something to make her not even want to try to repair the marriage and just cut ties and move on with her life.
You can never know who is at fault about what. That's why, despite my extreme contempt of backstabbing, I'm still trying not to judge (too much) and advice about not taking side in a breakup.
Conversely, that does not mean everyone is equally guilty (or guilty at all). You can't just assume the guy is necessarily wrong about something, just like you can't assume the woman is necessarily a cold-hearted female dog.