This app is sometime makes mistakes haha In fact u can write "that was her" or "that is her" is okay! Cuz Vietnamese doesn't always care about grammar construction for past, future and present tenses "Đó là cô ấy" can stands for "Đó sẽ là cô ấy", "đó là cô ấy" and "đó đã là cô ấy"
"That is her " is idiomatic but grammatically inconsistent with the rule that the verb "to be " takes a predicate nominative. "That is she" is correct, grammatical and actually used in speech. "That is her " has been called a "sturdy indefensible " because it is so resistant to correction in colloquial speech.
Why the minus 2? I answered as I was taught in school. "It is she" is and always has been correct. Some people even say ít in conversation. I usually do not but in an exercise I tried to be proper, only to be told that the grammatical answer was incorrect. DL should not be rejecting perfectly good English.
The problem with this line of reasoning is that language isn't stagnant. If people kept using what was considered 'correct', we'd all still be speaking in caveperson grunts. 'Correct' language is just whatever the majority of native speakers agree upon at any point in time. And because the majority of English speakers today would say that "it is she" is not grammatical, it is not grammatical. For all we know, in a hundred years, "it be dem" could be considered the most correct way of phrasing it.