Here you find an example that shows how to get initial value for assigned user to the user task. However when yo reassign the task this will not have updated information, since you rely on work item information and reassignment happens on task server.
Alternative approach is to define user that really completed the task as task variable so it can be mapped on process instance side again. That in fact would nice if the task server makes it accessible out of the box under a specified key inside result map that is sent over to process engine.
HTH