how to undo this or at least spin in his favor

[$tevie]

Another vote for not filling out "To" until the email is all finished and ready to send.

I just don't see this as a big problem. What he should have done was send the completed email with a note at the beginning "Pardon me. Hit "send" by accident on previous email". Who hasn't done this at some point in their lives? I must have received oodles of that type of message over my lifetime but since it didn't freak me out I can't say how many.

I wouldn't do anything now, since the corrected email wherein I would have put the explanation already went out. They can figure out what happened, and sending a third email to explain the obvious is emphasizing the incident too much, in my opinion. Let it go.