There is no easy way to unmerge customers.  You must manually reactivate the customer and then manually clean up the customer records.

1. Click Customer > Find and enter the Customer ID # for the wrongly merged (consumed) customer.  You can find this number by checking the History tab of the customer they were merged into.  Alternatively, you can search for them by checking the Inactive Account checkbox on Customer Find.
2. Edit the customer and uncheck the Inactive checkbox on the General tab.  
3. Cleanup the Contact Methods (address/phone/email/social) by manually deleting them from the customer they were merged into and adding them to your now reactivated customer.
4. Cleanup sales by using Customer Find to edit the customer they were merged into and click on their Activity tab.  Use the See Sale button to open each sale you need to move and use Supervisor > Change Customer to move them back to the reactivated customer.
5. If there were Wish Lists or Notes, visit their corresponding tabs, delete the entries from the merged customer, and manually add them back to the reactivated customer.
6. Save your changes when working with both records.