The cheapest way for her to stay on-site would be at Port Orleans in one of the rooms that has a trundle bed, There are 2 regular beds in the room plus a pull out under one of the beds which sleeps one. We did this once when we were a family of 5, now there are 6 of us. Most of the delux rooms sleep 5 because they have a daybed in the room, we stayed at the Poly once with 6 as my youngest was still in the crib. Renting DVC points is also a nice option because you get a kitchen and 2 bathrooms, but I'm not sure if this would be as cheap as a moderate hotel, may depend on what type of deal she can get. I do not think Disney has roll aways and I for one would never try to sneak an extra person in, I'm quite sure with MY luck I would get caught!!