To help with the constipation add fiber to her diet, which is whole grains, fruits, and vegetables. For whole grains, for example: oatmeal for breakfast, whole wheat breads, oatmeal cookies, bran muffins, whole-wheat pasta, and brown rice. Serve vegetables in any form, raw or cooked. Salads are great if she will eat it. You also need to increase her fluid intake with additional fiber. Fiber is great for increased bowel movements; however, it needs fluids to get through the colon. Suggest you limit bananas and fresh apples. They constipate, however, applesauce and apple / prune juice loosens stool. Her body had and is now recovering from chicken pox which, with any illness, causes the body to get out of balance. If the green stool continues to happen after another week, recommend going to her MD to get her checked.
My kids used to get bright green stool when they ate Fruit Loops or Fruity Pebbles cereal... if she has been eating that type of cereal it can also cause constipation. Also, have you tried magnesium supplements? Sometimes lack of magnesium can cause constipation too.