No, there is no setting to change the online presence for a user. If he hasn't been online, the status won't change. However, you should just send a message to the user because Viber's online presence status isn't that accurate. To be sure, connect to the Internet and then drop down the notification list and disable/enable Sync. It will do that automatically for all the apps that use the Internet to push for notifications. After this procedure, check the online status for the user.