I 2nd a wheel....S&W airweight hammerless is good....
I think that is a very bad recommendation.
A S&W airweight hammerless revolver is designed for pocket carry. The short sight radius and small sights that make it easy to fit in a pocket make it much harder to shoot accurately. The lightweight frame that makes it easy to carry significantly increases recoil.
I've got a 4" Model 66 (K-frame) and a 642. The K-frame is easy to shoot accurately and has little felt recoil with .38 +P. In contrast, the 642 is much harder to shoot accurately and, frankly, hurts to shoot. If your gun hurts to shoot, you aren't likely to practice with it often, unless you are EddieCoyle. Furthermore, a 642 is not an easy gun for a novice to master.
Since this gun is specifically for
home defense, a pocket gun is just a really bad idea. A full-size revolver like a 4" 686 would be a far better choice. A S&W M&P semi-auto in 9mm would also be a decent choice. A shotgun would also be a decent choice.
Skip the S&W Sigma -- they've had issues. As others have mentioned, if he's going to keep a firearm for home defense, he needs to get training and he needs to join a club and practice. The gun itself, without training and practice, isn't worth much.