I would
strongly recommend using #1 buckshot rather than #2 or #4 birdshot. Birdshot can be lethal, but the question is will it be
quickly lethal? I just saw a good quote on stopping power, "it's good enough to kill an elephant, but it's not good enough to
fight an elephant."
When you're trying to fight a man, as opposed to leaving him well enough to kill you before he crawls off to a gutter and dies, shotgun shot is the same as any other bullet, bigger is better! And #1 buck is the largest standard size which will fit 2 x 2 in the barrel.
If she's worried about the shot going through a wall, then she's just going to have to practice enough to put all the pellets on what she's aiming at. There are no shortcuts around that, because any birdshot larger than #6 will
also go through an interior wall and be potentially lethal on the other side, especially if a pellet hits a child (but once again, this is dying in the hospital type of lethality, not dead right there). And not coincidentally, birdshot sizes smaller than #6 are also very unreliable at killing people. It's not too uncommon for a healthy, large man to take a load of full power 12 gauge #6 or smaller birdshot square in the chest from a couple feet away, and then still be standing when the ambulance gets there. EMTs can have some real stories to tell about gunshot wounds! One specific one I remember, a guy was hit in the chest with a #6 high brass 12 gauge load from about 2-3 feet, and was standing on his front porch when the ambulance got there. Only one pellet made it through the muscle and bone of the ribcage, and it lodged in his heart. He still survived, though.
10 pellets of #1 add up to 7/8 ounce, which Goex recommends as the heaviest 20 gauge charge. 61 gr of FF powder gives 990 fps in a full size gun, or you can use 69 gr for 1010 fps. Or for a 20 gauge cartridge, they recommend 65 gr FFF, for 1100 fps.
I'd probably use 65 gr of FFF and a modern 20 gauge plastic wad (it may take some real force to get the wad down) with a shot cup, 10 pellets of #1, and an overshot wad. According to my very scientific estimates, you'll be looking at 850 to 900 fps of velocity. On birdshot, I would
not count on that stopping someone, but on 10 pellets of #1, it would do the job.
I've got the modern equivalent of the old howdah pistol, and it would be a real nightmare for a home invader. Same overall length, and modern smokeless shells result in about the same velocity (on a heavier shot charge because it's a 12 gauge). The more things change...