This friend, did you physically go on his server and attempt to build above 256? Because I edited the "max build height" in my server properties and nothing changed. (So why is that option even there if it doesnt do anything?)
I know I'm replying to an old post, but for the sake of the people who read this thread and keep wondering why the max height thing is in server.properties, it's for flexibility. Suppose an admin of a server just doesn't want people to make tall buildings or something. It gives you the option to restrict the height. It's not meant for augmenting it. It can only be set down.