Marking a task complete for a single user on a task with multiple users assigned.

I recently updated a task assigned to multiple users. I accidentally removed a user who had completed their part (i.e. previously selected "Done with My Part") and then reassigned the task to the same person. In doing so, it made it appear as a new task for this user, even though they had completed their part. How can I mark just their part complete if this happens again? To be clear, I am not asking how to mark the entire task complete. I am only asking about one person assigned to a task with multiple assignees. Thanks in advance!