I'd like to understand more clearly, you have "Shared" with someone who either had or created a new FormsCentral account (AdobeID), they are set as a "Co-author" and are able to sign into FormsCentral and view the form fine (which would indicate they are verified). You also added their email address in the "Email Notifications" section of the Options tab but you are saying it shows an "!" with information saying the user has to verify their email/account to recieve notifications.
Did I get that right? If the user can go to https://www.acrobat.com/formscentral/en/home.html and is able to sign in and use the application then they are verified. If they are not they will get a message telling them to verify and offering to re-send the verification email, this would resend the email they could then click to verify.
If the user is able to sign in and use FormsCentral but is still showing as "unverified" on the Notifications tab you could try removing them from notifications and re-add, or share the form with me at email@example.com and I can take a look further.