Co-author responds to email by mistake cc'ing the EiC

I think you can safely ignore it. I don't see how or why the editor would suddenly disfavor your paper because of something like this. They seem to think your paper is worthy of publication and they want to publish such papers, not get upset about small communication glitches.

And, of course, people often get cc'd on emails they shouldn't see, causing minor embarrassment. And I think that it is only that here: minor embarrassment.

Let it go. Someone got a chuckle out of it perhaps, but there isn't any reason for them to take out the big hammer.

I think this [is] horrible as formally the paper is still pending minor revision before publication.

It's not horrible, just a harmless glitch.

How should I react?

"Have you considered... Masterly Inactivity?"

I do not want to overreact

Good, because emailing the editor-in-chief would be an over-reaction...

_{PS - More generally - don't get so stressed out! You'll experience much worse gaffes in life to be worried and anxious about :-)}