Tami,
As far as I know, the interline rate is the same whether they book the week before or when it opens up. If they wait, they chance not going. I've never heard of a cheaper interline waiting until a week or two out, but stranger things have happened. We got ours a little over 2 weeks ago. With the Florida Resident, interline, and the castmembers all out there trying to get a big discount, I didn't want to chance waiting. A lot of times this is how Disney fills a ship. There is an interline desk at Disney she can call for the reservation, if she doesn't have it the number is 407-828-3111. At the time we booked, they had rooms from cat 5-10 available at a discount. She will have to prepay tips, but I like that being taken care of, more I can spend on other things on the ship.