Commas

Q. We use CMOS 14 and can find no solution to the following problem. “When we first met, he had done the unforgivable, and it had come out so naturally I’d been pleased rather than offended.” It seems the comma before and is unnecessary. Our author disagrees. Can you help?

A. I’m sorry I can’t check the 14th edition for you; I sold my copy to an antique collector after I tried to donate it to a home for retired copyeditors and nobody wanted it (they all had the 16th). CMOS 16 (6.28) is very helpful on the subject, however, recommending a comma in that position.