In order to execute a script (user event) via the sales order Approved button in view mode, the script execution context needs to be specified in the Event Type drop down list at the time of script deployment.
1) Under Customization > Scripting > Script Deployments, edit the script which needs to be triggered when the Approved button is clicked.
2) From the Event Type drop down (Under *status) select Approve.
3. ) Click Save.
Now when ever the Approved button on the sales order is clicked, the respective script will be triggered.