In the proof : If $A$ is separable, then $\Delta(A)$ satisfies the second axiom of countability. ($\Delta(A)$ the set of all complex homomorphisms of $A$.)
I have found they says "It is easy to see that the unit ball of the adjoint space of a separable Banach space is second-countable in the weak* topology."
But I do not find verifying it trivial, Need some hints to do it.