Hi there, wombatlawnandgar.
For now, emailed invoices will be tagged as "Viewed" in QBO even if they're opened by the copied recipient. The status is only reflected in QBO and will not be shown in the actual transaction that the customer receives.
I'll definitely send your feedback to our engineers so they'll take a look at this. Updates and improvements are based from the suggestions that are shared with us by other QBO users and customers.
Don't hesitate to visit us again next time or post a question in the Community if you need help.