。
She's abstaining from alcohol.
Literal
She [topic-は] alcohol [object-を] is-refraining-from.
慎む (つつしむ) means to refrain from, abstain, or exercise restraint — a formal, somewhat literary verb that implies self-discipline. 酒を慎む is more elevated than the casual 酒をやめる (to quit drinking). The ~ている marks this abstinence as an ongoing state — she's currently maintaining her restraint.