Which handgun would you recommend to someone with little or no firearms training or experience, for home protection?

Personally, I do not agree with using a hand gun for protection at home. I use a .12 gauge pump shotgun (Remington 870), because my wife, and children , have no problem pointing , and shooting it. Plus if you don't wish to kill the intruder, the sound it makes racking a round into the chamber will USUALLY intimidate the hell out of a criminal.

If he's in California, he should just start with the approved list of pistols so he doesn't end up in prison for being confused and thinking he's "free".

None, until they get some training and range time under their belt.

I agree with SPC Elijah. Except, I prefer the short barrel and pistal grip. It is better in close quarters like ships or homes and in low light. Also, defense should be a layered approach. Fence, big wolf dog/s, locks and support can be 911.

I'd go with some form of 9mm and load it with hyrdoshocks. A 9mm, say a sig, is eminently controllable, even for someone with little to no experience. Additionally, they are easy to wield, easy to upkeep, and far more practical for moving around a house than a shotgun. Finally, the hollow point ammunition will do more than enough damage to an assailant for purposes of defending house and family while preventing the likelihood that a round will go through a wall and inflict unintended damage.

If they’re set on a handgun, I’d recommend a .357 revolver. Simple to operate. No clearing procedures if you have a misfire. You can fire .38 special, .38 +p, or .357 through it depending on how comfortable you are with it. With some good speed lifers and practice they’re easily reloaded. Personally I still feel a pump action shotgun is a great home defense platform.

I think I would recommend a revolver chambered to hold 410 shot gun shells loaded with buck shot.

It's actually more of a "two hand" gun, but it's impressively easy to place large holes whereever the red dot appears. Recoil is noticeable but not a problem as long as you're not holding it near a face or shoulder. Practicing aiming and firing from the hip is actually fun (unlike practice sessions with a 12 gauge).
And it could even work well as a club when the yellow things have been emptied out.

Edit: For places where this "shorty" is frowned upon, the "Bantam" model 500 in 20 gauge, with an 18-inch barrel and after-market pistol-grip is only a few inches longer, but the pistol grip makes the recoil noticeably more apparent.
Edit2: I do mean aiming when firing from the hip. That's a Crimson Trace laser fitted on the receiver, so you have a little red dot showing almost exactly where the column of buckshot will go.

Photo of Crimson Trace laser spot on picture frame.

A Glock 42 or 43 (.380 caliber or 9mm, resp.). Order it equipped with a Crimson Trace laser. There’s no external safety to fumble with and the laser allows the shooter to quickly acquire the target and fire an accurate first round without even lifting the weapon to eye level and looking down the barrel. By keeping the laser armed at all times, there’s no need to fumble with its on/off switch on the left side of the grip; simply squeeze the laser trigger with the underside of your middle finger and fire the weapon as normal with your index finger. I practice hand-and-eye coordination in my home office while seated at my desk by dry-firing at the corner of a picture frame hanging on the wall about ten feet away.
Photos show the Crimson Trace laser housing on the upper edge of the right-side grip on my Kimber .45, with the vertical on/off switch in the left-side grip and the two halves joined by a black strap behind the trigger guard; the strap conceals the laser button that is squeezed by the underside of the middle finger. I’ll try to photograph the laser spot on the corner of a picture frame in a separate post.