坐观成败

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 sit and watch, but remain indifferent (idiom)

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 from the sidelines and do nothing (idiom)

to turn a blind eye to