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InfoPath - Get user information without writing code (extended)
Also, as a hint, another method that may be more straightforward is to use GetCommonManager, because that accepts the same value as GetUserProfileByName, but automatically returns a subset of Manager info without having to re-query the GetUserProfileByName web serivce. You can set it to automatically retrieve data upon form load and then auto-populate your Manager fields just like your current user fields.