If he hasn't yet used the ID for anything, the easiest is to forget it and register a new ID.
He probably can't have done much, as you can't verify an incorrect email address.
Otherwise, you should still be able to log into the Adobe website using the email address, even if you can't send emails to it.
Goto the https://www.adobe.com/uk/account/account-information.html page, and enter the correct email.
That will leave the ID the same, but just associated with a different email address.