How to reset email.

How to reset email.

My colleague created a new CRM account and then deleted himself as an admin/user.  I then went in and created a separate, new CRM and put in his email address to add him as an admin/user and I received the error: "
This Email address is already taken. Specify a different Email address for the user."
 
How can his email address be "reset" so that I can invite him to the correct CRM?

Thank You

James