I think an x ray of the arm should be taken, as well as a scan of her brain to ensure it's not a neurological problem. If there's no physical problems with the arm after the x ray, it would be a good idea to see the physiotherapist, it could be that her muscle tone in the arm is weaker than the other and therapy could help tone her up. Usually people have a weaker muscle tone in the arm they use less, because it is not being regularly strengthened by daily activities as the other.