I'm sorry to hear of your loss. I wish you the time to grieve and one day heal.
As to his filing for a divorce, that's meaningless. Her argument will be that they were reconciling. The mere filing of divorce documents doesn't mean a darn thing.
Dad might have regretted it, and was going to do nothing. Dad might have wanted to reconcile. Unfortunately, you'll never know to arrive at a legal conclusion.
Bottom line, she's his legal heir. The length of the marriage isn't relevant.
The filing of divorce papers isn't dispositive or probative, insofar as a court would be concerned.
Talk to a lawyer or two, but don't get your hopes up too high!