Firstly, your friend could easily be two weeks late giving birth to her baby. Secondly, even if she has her baby on time, she will be too tired to be walking around Disney when she just had a baby 3 weeks before. Thirdly, I would not take a very young baby on an airplane unless absolutely necessary because airplanes recirculate the air and if someone on that flight has a cold or bronchitis, the baby could get very sick. A young baby just doesn't have the immune system to fight a simple cold like older children and adults -- a virus that gives an adult a cold could very well turn into pneumonia in a young infant.
Your friend should wait until the baby is older -- she should ask a pediatrician for a recommendation, but I would say 6 months or older since a baby can't even use sunblock until 6 months.