Will that user only have edit access to that particular node and all of the children? Or will they need access to a separate node at the same level or higher and all their children too?
I asked this because the singular setup is already enabled out of the box. On the user's record, if you define the starting path to a path in the content tree, it will only display and allow them to edit that node and the children of that node.
If you want to restrict to two separate notes then you need to use rolls and permissions.