You cannot subscribe for a different user. As the docs for the method say, the base JID must match the user making the request. You can only change the resource.
What do you mean by “You can only change the resource”?
So what can I do in order for one user to publish some information in one node in his server, and for the other user from other server to subscribe to this node and receive updates (pubsub)?
Actually, my first statement was incorrect. If you are the admin, or the node owner you can subscribe other people. In you case though, why isn’t the other user subscribing themselves?