Currently, there is no way to see what changes your colleague made without simply comparing your original to his new published data source. Essentially, when he creates a local copy and republishes it with his own calculations he is creating a new data source. Therefore, it is not possible to roll back to the original since Tableau considers his newly published data source to be the original.
Hope this makes sense!
Tracy, thanks for the answer. Take a look at what we've "invented" in our company. We have SVN repository which allow us to support versions of each file saved in this repository. And when report developer is done with his tableau report in desktop, he just saves his Data Connection as a tds file in this repository. (Data->Data Source Name->Add to SavedDataSource) This action saves file to MyTableauRepository BUT you can change file destination each time you save it. And then we have tds file which can be opened by anyone with Tableau Desktop, he will see all calculated fileds from it , and if some changes are done - we have version control supported by SVN! Maybe this would be helpful for someone!