坐收渔利

the benefit reaped by a third party when two sides are locked in a dispute

lit. sit on the mountain and watch the tigers fight (idiom); fig. watch in safety whilst others fight then reap the rewards when both sides are exhausted

to reap where one has not sown (idiom)

to sit and await success or failure (idiom); to wait to see the outcome of a fight before taking sides; to sit on the fence

lit. when the snipe and the clam grapple with each other, the fisherman catches them both (idiom); fig. when two sides are locked in dispute, it's often a third party that benefits

to fish in troubled water (idiom); to take advantage of a crisis for personal gain