。
She's off duty tonight.
Literal
She [topic-は] tonight [topic-は] off-duty is.
非番 — literally '(not) + (turn)' — is a workplace term for being off the duty rotation. It's the natural choice for shift workers like nurses, police officers, train drivers, or firefighters, where the schedule alternates 当番 (on duty) with 非番 (off duty). It's not the everyday word for 'a day off' (休み or 休日); 非番 specifically means it's not your shift today, even if your colleagues are working. Note the double は: 彼女は marks the sentence topic, while 今夜は contrasts tonight with other nights, hinting that things may be different on other days.