I think the Scott Kelby Digital Photography Book series fits the bill here. These aren't books that I really enjoy or appreciate myself as technical person, but I know they are technically accurate, and they intentionally aim for an approachable, non-technical style. The "elevator pitch" for the series is:

If you and I were out on a shoot, and you asked me, 'Hey, how do I get this flower to be in focus, but I want the background out of focus?' I wouldn't stand there and give you a lecture about aperture, exposure, and depth of field. In real life, I'd just say, "Get out your telephoto lens, set your f/stop to f/2.8, focus on the flower, and fire away." You d say, "OK," and you'd get the shot. That's what this book is all about. A book of you and I shooting, and I answer the questions, give you advice, and share the secrets I've learned just like I would with a friend, without all the technical explanations and without all the techno-photo-speak.

This is a way your friend might learn some of the technical details without approaching them from an engineering direction.