Hello Larry,
On the top of your Cvent account you will see Contacts (next to Admin).
Find the client profile in question click the Edit button (green button at top) and then scroll down until you see Opted-Out. Now change this to "No" (it should show as "Yes".
Make sure you receive permission from client before changing this setting. Also, make sure you confirm the email address is correct. IF the email address is not a real email, Cvent will automatically mark the contact as Opted-Out.
Good Luck, Larry.