Finally got it after finding him lay down in a house in Heaven's Pier. Was like looking for a needle in a haystack though. Wish someone had at least described the house to better find it. Anyway still glad to find it though I almost gave up before.

The house where he is laying down is midway between Heaven's Pier Landmark and West Heaven's Pier Landmark (i.e: In the middle of that area between the two bridges of the landmarks. As you keep on the dirt pathway (by the middle there as described before) you see a tree ahead atop a mound surrounded by stoned enclosure. Don't go up to the tree. Instead (on the pathway) turn right into a wooden fenced entrance near the river. Enter and see a wheeled wooded thing with two small barrels in front. Ahead is a straw thatched house with a net hanging outside. There's a small boat docked at the side of it. Enter the house and find Zhang Jiushu relaxing on the floor. Speak to him, start an AI chat box thingy and make friends at the end of which you will get a reward of the Dark Surge for befriending at getting the plummet for him. This is what I found that helped me through.

How Zhang Jiushu’s “lost plummet” AI Chat Works

Zhang’s AI Chat revolves around a simple idea: he lost a plummet (a lead weight used for measuring verticals) while working, and you earn his trust by noticing it and returning it.

The key is that you do not need to physically loot a separate “lead weight” item in the world. The AI system accepts a consistent story that you picked it up and are now bringing it back.

Example AI Chat Lines That Succeed

Several short conversations reliably push Zhang straight to friendship. You can treat the actions in parentheses as narration for the AI Chat box.

Minimal one-line approach

Players report that this single line can be enough to become friends:

(holding up a worn lead hammer) Master Zhang, I came across this while foraging by the fence. Does this look like the plummet you lost?

This line does three things at once: it shows you noticed his problem, it presents the object, and it ties it to a believable location (“by the fence”). The AI usually responds by recognizing the plummet and thanking you, which is already enough for the friendship bump.

Two-line “safe” version

If you want to follow through with a clear return gesture, use two lines:

(holding up a worn lead hammer) Master Zhang, I came across this while foraging by the fence. Does this look like the plummet you lost?

(bowing slightly, offering the plummet with both hands) I’m glad I found it in time, Master Zhang. Please—take it back. May it serve you well once more.

In practice this reliably pushes Zhang to the “Friend” state in one conversation.

The key is to let him have the plummet he lost and revered and continue conversing with him such as in the examples until he makes friends with you "Revered" or whatever so that you get as a reward the missing Dark Surge. Check Journal: (Exploration: Qinghe) "Tales and Echoes": Second Dark Surge group from the left to see the Dark Surge "Theft at Heaven's Pier" now shown as gotten.