坐山观虎斗

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

benefit from others' dispute (idiom)

to watch the fires burning across the river; to delay entering the fray until all others have been exhausted by fighting amongst themselves (idiom)

to watch with folded arms (idiom); to look on without lifting a finger

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

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