a small amount of swelling is quite normal. it is caused by immobilizing the wrist and hand. the hand and wrist movements are like a pumping action to assist the heart in circulating the blood. imobilize the wrist and elbow and you are effectively reducing the effeciency of this system. Also a fracture is trauma to the bones and soft tissues and so generates swelling from bleeding.
advise your wife to keep her arm elevated when at rest or in bed. eg on pillows this should help.
if there is a lot of swelling to the extent it causes pain or compromises circulation then your wife should see her doctor again as the plaster may be too tight and need loosening.
advise your wife to move anything in the effected limb that is able to move if not immobilized. ie shoulder, elbow if not immobilized, fingers. this will pprevent some stiffness and so speed recovery after removal of plaster. there will probably be space for a very small amount of movement within the plaster and this is ok too.
good luck