Unless the owner of House A either "knew or should have known" that the tree was in danger of falling down, it would be considered an Act of God, and the owner of House B would file it under their homeowners policy.
If the insurance co. for House B wants to investigate further as to liability, then that would be their option, but, depending on the total amount of damages, I don't know that they'd pursue it.
Do you have some reason to believe that the owner of House A knew that there was some sort of problem with the tree? Was it diseased or damaged previously?