You know, a vague poll such as this may be about as relevant as helping pick out underwear for your buddy via such a poll. No doubt you will get some answers and maybe even a consensus (but probably not), but that does not mean the results will fit your friend's situation.
Me, I like the Rem 870 (18" barrel) solution with an extended mag tube and loaded with 00 buck. For me, I live in the city on a fairly good-sized lot with the greatest shooting distance fro.m my home to the property line being 30 yards. Even with the shotgun, it is not what I have with me most of the time, however. It remains as a staged gun. That means it has limited HD potential if I am not near it. Thus, much of the time I have my regular carry gun with me.
If I had a smaller lot or lived in a trailer or apartment, I might opt to go with #4 shot or even a heavy dove load if I have a lot of concern about over penetration to the neighbors.
If I lived in the country with many acres, I might be more inclined to go with a rifle like a .308 since I would be less likely to have a problem of errant rounds hitting neighbors and because engagement distance might be greater.
For most folks, you can't really beat the destructive force of a 12 ga. shotgun inside 25 yards. However, it is important that you pattern your particular home defense load at various distances such that you know how much spread you are going to get with your load. People seem surprised to learn that they can miss with a shotgun or just how small the pattern may be for a shotgun with a barrel as short as 18". Out of mine and using Federal 00 Buck Tactical load, I get about 3" of spread for every 5 yards of distance (checked at 5, 10, and 15 yards. Inside 5 yards, there is just going to be a very nasty large hole. At 15 yards and shooting COM, the perps chest should be peppered with a smaller dinner plate pattern. If you only get a chance with one shot, you get a lot of bang for your buck (shot, buckshot that is) with a shotgun. While it is a short barreled long gun, even at 15 yards, the gun still has to be sighted correctly to assure a proper hit.
If you go with something like a heavy dove load, the pattern should open up more quickly than 00 buckshot. I did pattern birdshot at one time, but seemed to have misplaced those results.
Another neat factor to consider is that if you don't have a choked gun, your shotgun can be used as a shotgun or as a slug gun. A 1.0 or 1.125 oz 12 ga. slug traveling 1200-1500 fps will produce a significant wound and can be used at greater ranges than bird or buckshot. As with patterning of shot, you need to take the time to sight in the slug rounds to be sure you know where they will go relative to the shotgun's sights. At 50 yards, mine shoots 5" low and and about 1" left (not trigger jerk as verified by other shooters) from point of aim.
Something else to keep in mind for a home defense gun is that both your friend and his wife need to be familiar with the operation with the weapon, practiced, and have no problems with handling it. The gun may not be much good if one partner is too afraid of the gun to use it. If the 12 ga. is a little too much, then you might consider a 20 ga.
As for the query as to how you answer the door with a shotgun, the correct answer is, "Any darned way you want to!" The point of the query is a good one, however. No one gun will work optimally for all home defense situations. It would appear that you can get around better with a handgun and the handgun may be more convenient for at home carry, but it doesn't have near the stopping power of a rifle or shotgun.
The fact is, you won't and you will be unarmed. There will be more cases where you will grab the pistol and at least be armed than leaving you long gun in the closet.
