Hi Scott,
Does this customer show in your Resolve Conflicts? To check, click the gear icon in the bottom right of the grid for the Customer list. You'll see checkboxes for the grid fields. Check the Entity RecordID at the very right (see screen shot) and click Update. Then go to your QuickBooks > Resolve Conflicts and check for the entity recordID of the customer. If it shows, try a refetch from the Resolve Conflicts screen. This may bring the record back.
Also check QuickBooks to see if the customer exists there also. Did you try to delete this customer at some point?
- Adam