At a GUESS, the fungus may not be what is killing the tree, but just something that is colonizing the limbs because they are dead. The fungus can be part of the process of breaking the dead wood back down.
I would try breaking some twigs on the part that looks non-impacted to see if they are still flexible and green inside.
Is the fungus only on limbs or is there some on the trunk as well? That would be a very bad sign as to whether the tree can still be saved.