"dela" is a conjunction of "de"+"ela". It can be used as a possessive pronoun ("her car" = "o carro dela"), or when the verb used before the word "ela" requires the "de" preposition ("eu gosto dela" = "I like her"; "eu gosto ela" is wrong).

So in this case, "ela" is used if you refer to the person ("Not her."), and "dela" is used if you refer to something that she can possess ("No, not hers.").

Dela = her(s)/ of hers. Ela = she / her. Dela would be "not of hers"

"Não, não ela" is also right. It is being accepted now.