Yes, I see what you mean. And of course, Duncan is kind of playing John here, in the sense that he's getting him to do what Duncan wants (and needs!) him to do -- but it is nonetheless sympathetic and self-aware.

The problem, as you say, is that Duncan will want to involve himself, and John won't like that -- but Duncan's main concern here isn't John's, exactly. Duncan wants to know what Meg is (did she survive the fall because she was possessed, or because she was immortal? the second would require Duncan to get involved) -- whereas I think John assumes that the demon possessing Meg will ditch the body and move on. So he might be willing to let Duncan persuade him to let him clean that up (from John's perspective!).