Unless the new shooter is physically too small/weak to handle a 12 guage, the previous posters are correct. Usually we consider 20 gauge because of the impression that the recoil and the weight will be reduced. The weight of a 20 ga is, in fact, usually lower. But, that's the very reason why they are not the right choice.
Felt recoil is a factor of the energy released by the shell vs. the weight of the gun. When we use a lighter gun, such as a properly scaled-frame 20 ga., we actually INCREASE the felt recoil by reducing the weight of the gun absorbing the energy from the discharge of the shell. If the shooter is about 15 yrs or older, they can usually manage the weight of a 12 guage. Using the lightest loads available for the 12 gauge will give them a softer-shooting gun with a generous pattern to help build confidence.
Also, while upland hunting guns should be light and lively to enhance quick pointing, clays guns should be heavier, to promote a smooth swing and absorb more recoil energy. The snappy recoil of a light field gun isn't as much of an issue due to the lower number of shots taken and the longer recovery time between shots in the field. Conversely, target shooting involves higher numbers of shells fired and much shorter times between shots.
ALWAYS encourage new shooters to hold the shotgun tightly against their shoulder. Once a bruise gets started by loosely mounting the gun to the shoulder, every subsequent shot painfully expands the bruise and detracts from the enjoyment of the experience.
I always recommend single shot guns for beginners (or loading only one shot at a time in repeaters). This has two benefits - better safety (they can't spin around with a loaded gun to celebrate a hit) and building better habits (not relying on the second shot). Where recoil is especially a problem, gas-operated semi-auto shotguns can be used to absorb even more energy.
Double triggered doubleguns are the best choice in the field, primarily for instant choke selection, but single trigger doubles offer faster, more consistent second shots at clay sports.