Hi Lisa

The course completion editor was introduced in version 9 to help remedy these types of issues - if you can upgrade then this would allow the record to be edited.

If this isn't possible to upgrade then I can't think of a way in the interface to delete an individuals completion record 

When an activity's completion criteria is changed there is an option to keep existing completions or delete existing completions and recalculate the completion based on the new conditions. You have chosen to keep existing completions - if you choose to delete then the activity completion date would be recalculated to when the completion is recalculated. These options are for all users rather than individual users.

Regards