The use of Path to the Grave uses the cleric’s Channel Divinity ability and does not require the additional rules around concentration that spells do.
Taking a Ready action to use their Channel Divinity does not require concentration because it is not a spell. This is simply an action that is awaiting a trigger that is no way different than readying an attack or a movement. They haven’t actually ‘done’ it yet, they are waiting on a trigger to do it. They can either get their trigger and use their action, or the trigger never comes, or they opt not to do it for some reason.
But there is no concentration involved, so moving forward I would recommend discussing what happened and what will happen in the future (no concentration checks).