I personally like a little chaos, I would just respond to the original email. Then like one day later respond to the coworkers email with something like "Taken care of yesterday! see below" and copy the email from the day below.
I feel like at that point it puts him in a tough spot because nobody cared about his stupid email to begin with, and if he wants to respond now he has to write out a whole longer thing about how he sent the email before yours, which also nobody is going to care about.