I am sorry to hear of this frustration. Is it possible that this contact forwarded the email and the second recipient opted them out?
Unfortunately there is no way around this due to Can Spam and CASL laws. They must re-subscrube from their end. Have you asked them to enter their contact information into a JMML box on your website or Facebook page? If you don't have a signup box on your site you can send them the link from within your account.
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