You may find that the 20ga kicks more than the 12 due to the lighter weight of the gun.
If she is able to use the 12 try the lighter 1oz loads with a correct length stock and good recoil pad, options like recoil reducers are also an option.
If she is able to use the 12 try the lighter 1oz loads with a correct length stock and good recoil pad, options like recoil reducers are also an option.