Has the wife tried different weapons, or is her preference based upon impressions ? Even a short barrel 12ga can be cumbersome for some but Remington makes a 20 ga pump in a youth model, with a shorter stock and barrel. I've seen new ones for $225 on sale. Federal makes something like #3 buck loads in 20 ga, which would be a good candidate.
Whether you get a shotgun or handgun, revolver that fits her hand recommended for a handgun (S&W has better finishing / fewer sharp edges than something like a Ruger), I'd also recommend a laser sight of at least a flashlight set up so that shots impact center beam. Some time at the range can sort some this out, where you'll find that the shotgun with flahslight / laser is most effective, then handgun with flashlight / laser, then shotgun open sight, then handgun open sights. I've seen people empty two high capacity magazines from a 9mm pistol at a 25 yd station, miss all shots and have to move on during practical shoots, while even my kids can hit things with the shotgun.