"The ‘is’ is like an equal sign in this instance, because sentences like this, “is” shows the state-of-being relationship between the two ideas. “She” is identifying herself in this sentence. ‘This’ and ’she’ are the same thing, and therefore are in the same case (the nominative)."

in person, we point to someone and say "That's her" not "That's she" is that wrong too? should we say "That's she?" or does switching "This" with "That" change the structure of the state-of-being relationship between the two ideas ? is it because she, instead of someone else, is identifying herself ?