My recommendation
Most of the foregoing are spot-on. Long gun and revolver recomendations especially for someone reluctant to do a lot of training.
By the way, a firearm (especially handguns) will encourage training only if they can produce good groups. Good groups are VERY encouraging to the practitioner to continue practicing. Random hits with little ability to control the points of impact are very discouraging (in my opinion and experience).
I will start off-topic and bring it around to your original question towards the end.
First, non-gun alternatives might be your friend's best alternative. (I don't recommend pepper spray inside a home, although the decontamination -no matter how expensive- would be cheaper than any lawsuit wrongfully brought to your doorstep.) The prior suggestions of .38 Special revolver are good. Not too large and not too muych recoil, but the .38 has a long history of discouraging intruders and wrongdoers.
However, for anyone interested in defending a home, start where the efforts produce the most effect. Prevention.
First, create defensible space around your home. No hiding places, good lighting secure windows and doors that cannot be breached with anything less than the "jaws of life". There's more you can do with passive defense, but you get the idea. Preparation is key.
Then an early warning system. The only thing better at sounding an alarm at a stranger's approach than a good, protective dog is a couple of geese. They honk at everything new. And you can't bribe a goose with a hamburger. Unfortunately, you can't housebreak a goose, either.
However, a dog has other advantages.
As a tool for home defense (and even away from home, too), a dog in the 75-150 lb range is great. The right species (of which there are many), will be good with kids, protective of the home (whether you are home or not), intimidating to evildoers even on the street or in a parking lot, yet not so alarming to anti-gun types. They make good burglar alarms that never need batteries. They also have other benefits, like; They enable good exercise, are excellent psychiatrists, and allow the children to learn about raising a being who is dependent on them, unselfishness, responsibility, loyalty, and a different kind of love than family bonds. Just remember, that the amount you practice with your handgun, you should also invest in training with your dog. They will love you for it.
One more thing. You can warm your toes under a sleeping dog. Not a good idea under a gun.
In firarms, generally, the most effective gun choice is a short barrelled long gun. Something two-handed with a stock is MUCH easier to hit with. If you are (your friend) is taking a stationary defensive position (see "safe room" below) mobility is not an issue. For the recoil-sensitive, a 20 gauge (or the much ignored, so-called "sweet sixteen") 16 gauge shotgun is an excellent choice. Both are lighter than a 12-gauge and less recoil, but the action is the same as 12-gauge, so familiarity is transferable between arms.
For home defense, get yourself and your family into a "safe room" where you will be keeping a 12-gauge pump action shotgun. There is only one thing more compelling toward a hasty exit than the sound of a pump shotgun's closing bolt. If that sound comes from behind a closed door, no sane criminal will ever open that door. And if he does, an 18" shotgun is a lot easier to shoot straight and true than any handgun. The right loads can also be selected that won't penetrate your exterior walls to endanger your neighbors.
It has been said many times (so much so that I do not know the original author) that a handgun is what you use to fight your way back to your REAL weapon. Remember, a short-barrelled long gun with a stock (none of those pistol grips) is MUCH easier to hit with.
I will steal a line from someone else I have seen on these forums. Sorry, I don't have the name handy to give full credit. "If you find yourself in a fair fight, then your tactics suck." Prepare beforehand, so you will never have to be in a fight at all, much less give your opponent a fair one.
Right to self-defense generates an obligation to train enough to be responsible in its use and careful consideration of one's willingness to use that force. And remember, even a dog brings responsibility to your doorstep. What brings more responsibiity? A gun or a dog?
Thanks for reading.
Lost Sheep