。
She's wrong.
Literal
She [topic-は] is-mistaken.
間違っている ('be mistaken / be wrong') uses ~ている in its 'resulting state' sense — 間違う is a momentary verb of erring, and ~ている here marks the lasting state of having erred, not an ongoing process. So the sentence describes a current state of being wrong rather than the act of making a mistake. English doesn't have a clean equivalent: 'She is mistaking' would be wrong; 'She is wrong / She has it wrong' captures the state. This is one of the trickiest aspects of Japanese ~ている — whether it reads as ongoing action ('is doing') or resulting state ('has done and is in that state') depends on the verb's lexical aspect.