how to undo this or at least spin in his favor

[Beam Me Up Scotty]

"I recommend he tell them the truth - he accidentally clicked the send button before he had completed the email"

Yep, honesty is the best policy (at least in this case).

I like the Gmail feature "Enable Undo Send" where you can cancel an email sent by mistake. I've used it on a number of occasions. For those who don't know how to turn it on; it's in the settings/general tab.

D & C