I think rules as written you no longer need to be adjacent to the enemy you are helping your ally hit, or even alive. Same goes for in aiding with ability checks, like in the example of climbing the tree.
I think it depends on the narrative of how you say you are helping, if you describe that you are cupping your hands and going to give your ally a boost up the tree, you are readying your action to help them. If you shouting encouraging words like "You can do it!" from across the room for him to climb the tree than readying the action doesn't matter. Same could be said for the action in combat most of the time it is just you distracting the enemy, I could see the distraction coming from someone who was adjacent a moment ago but is now lying dead on the ground a few yards away.
But, saying all of that, I think it would be not unreasonable to house rule that the Help action must be readied with the trigger of the character you are choosing to Help turns starting.
I think it depends on the narrative of how you say you are helping, if you describe that you are cupping your hands and going to give your ally a boost up the tree, you are readying your action to help them. If you shouting encouraging words like "You can do it!" from across the room for him to climb the tree than readying the action doesn't matter. Same could be said for the action in combat most of the time it is just you distracting the enemy, I could see the distraction coming from someone who was adjacent a moment ago but is now lying dead on the ground a few yards away.
But, saying all of that, I think it would be not unreasonable to house rule that the Help action must be readied with the trigger of the character you are choosing to Help turns starting.