I have some information for you!
After checking the mail logs, here's what appears to have happened. Your client's customer received an email at 12:12 pm on 8/2/2016. A few hours later they received an email confirming that they unsubscribed, and have not received an email since. Is it possible they were complaining about the message confirming they unsubscribed?
Also, whenever a customer unsubscribes from a newsletter list, the tags are removed from that contact's CRM record, so that would be why they have no tags now, but did receive the email earlier.