Any way to get notified after last email in a series is read?

I have a client with an introductory email series, 6 emails, spaced out over 10 days. We'd like to find a creative way to get notified (ideally by email) when a customer has opened email #6. I know we can see it in the dashboard and reporting tools, but they don't want to have to check manually, becasue the notification would go to their call center.

Any suggestions?