This must be very shocking, not only to your M., but also to you and your children. It may feel like your whole foundation has been pulled out from under you. That's how I felt when my parents divorced while I was in college, even though I had feared it might happen for a few years beforehand. A person's vision of her "world" changes in this situation more than anyone can possibly understand without living through it yourself.
Twos thoughts to share with your mother (coming from a college professor who teaches a course in marital communication):
(1) She may be more hurt and upset about HIM being the one to leave, and thus, rejecting her, than she is about LOSING HIM. Once she steps back and looks at the situation objectively, she may find that she doesn't miss him at all. She may be relieved to live in peace without him. If she hasn't lived on her own in decades (or ever), that is unnerving. But, she may end up loving it. No one will be arguing about how she does things, she won't feel resentful keeping house for an ingrate, or whatever else they fought about.
(2) If there is any hope for the marriage to be restored, they must completely revamp their way of relating and thinking about each other. It isn't over until the divorce is final, and even then, it doesn't have to be over. Buy her and your father each a copy of this book: "How to Improve Your Marriage Without Talking About It" by Love & Stosny. http://www.amazon.com/Improve-Marriage-Without-Talking-Ab... This book will help them to recognize the pain they have likely been inadvertently causing for each other, as men and women usually do not understand each other's greatest vulnerabilities. They'll also be able to understand their own reactions and root causes of behavior. For example, they will both be able to understand your mother's sense of desperation in the aftermath of having a woman's greatest fear take place: being abandoned. Your father may have had his pride stomped upon for years during their conflict, when all she wanted was to feel acknowledged and important to him, yet couldn't express it in a way that he didn't leave him feeling criticized.
Not getting along often means that they had a lack of connection, and this book helps people discover how to restore or build connection. I know of several marriages that this book has saved. The title appeals to men. (Hint: Put it in the bathroom where he can read it without you seeing him.) At the very least, they will both be better equipped to have healthy future relationships, and end the marriage recognizing why it went wrong.
Remember that all of you are grieving. What died is their marriage and your conception of your family of origin. When you lose a loved one (or lose your intact family), to death or divorce, you never get over it. It will never completely be free of some vestige of pain and regret at the loss. However, you can carry on with your lives, and re-shape your future to be better than the past. I wish you all the best.
Diane