Your user needs a working email address to communicate through Nabble.
You can change his both your user's Nabble password and email address. Either way he needs a working email address.
If a user doesn't know the password of his email account he needs to create a new email account then tell you what that address is and you can then edit his his profile to show his new address. That generates an email confirmation email with the link he needs to click to confirm his control of that address.
Just a Volunteer Nabble Helper - because the nice folk at Nabble have helped me! GregHelp - Building a set of answers to Nabble FAQs.