Hi James

If you go to the completion editor  and edit the completion record for the user that is unenrolled there should be a transaction record of the activities completed.

Note  

• Unenrolment deletes grades from the gradebook. If you are using audience sync to unenrol users there is an option under 'external unenrol action' 'Disable course enrolment and remove roles' which retains the grades but doesn't allow the unenrolled user to access the course 
• Grades that are overridden in the grade book won't show 
• Some activities store their grades separate to the gradebook (quiz and assignments) and are recoverable
• there is an option to recover user's old grades if possible when you select enrol users

In addition, in General settings, there's an option for "Recover Grades Default" that you can check.

Then, you can re-enroll the user to view the grades.