。
She's already back.
Literal
[topic-は] already returned-is.
戻る ('return, go/come back') is a punctual, change-of-state verb, so 戻っている describes the resulting state — 'is in the state of having returned' = 'is back.' The same ~ている construction with a continuous-action verb (走っている = is running) reads as ongoing motion, while with punctual verbs (結婚している = is married, 死んでいる = is dead) it expresses the resulting state. もう ('already') flags that the change happened earlier than the listener might have expected.