Sounds like your client has an increased amount of fat cells in her arms which is genetically inclined as the other two stated. Increasing your clients overall daily energy expenditure can help shrink those fat cells in her arms. Also like the others stated diet is also a major factor. Increased energy expenditure and lowering calorie intake will keep your clients body burning those calories so extra fat is not stored in those fat cells in her arms. I would incorporate a lot of calisthenics in her sessions and long with bicep and tricep exercises. Also train the shoulders and back because those muscles attach to the arm as well especially to target the upper back part of her arm. Push ups, assisted pull ups, lateral and front raises, lat pulls, maybe incorporate some shadow boxing exercises, and of course bicep and tricep exercises.