It depends how the headers were populated. It is possible to include both a recipient name and a mailbox specification in RFC 822. If it was addressed like:
Jane Smith then one could argue John Doe should not open the message.
As crazy as it sounds to do something like this on the Internet, there is precedence in some messaging services. Skype, for example, lets you delete messages after they have already been sent. Obviously there is no guarantee that the other person didn't read it yet, but if not, then it is effectively unsent. If it had been read, the other user has to go by memory unless they copied and pasted the email.
Google has the ability to do the same thing with mail sent to a GMail account in many situations. IMAP clients will tend to sync up and would likely go and remove the message at the next sync. If the user had not read it yet, it would be effectively 'taken back'. If the user had opened and taken a cursory glance at it they would in essence accomplish the same purpose. If the user has a POP client they are obviously out of luck.
What I wonder though is what sort of data it is and whether this is a sole measure of protection or an additional measure. Depending on how quickly they discovered the error and got a hold of Google, there is likely a very slim chance the message wasn't read at all. Further, they have no guarantees that the message wasn't copy and pasted or otherwise stored. Perhaps it is a best effort type of deal.