DCL handles this and all other food issues marvelously. All she needs to do is to notify the head server on the first night and/or have a note added to her reservation. At buffet meals, the head server and/or chef will walk thru the line with her to advise her on safety issues. At sit down meals, they will prepare whatever she wants in a gluten free fashion. If you plan to go to Palo, notify them a day in advance, and special treats will be waiting for her.
The biggest issue on the ship will be that she'll get spoiled rotten!

She won't get this kind of service in a non-Disney establishment.
Seriously, she may have a problem if she chooses to eat off the ship (other than at
Castaway Cay, where all will be handled for her). The easiest way to handle the issue is to eat all meals on DCL property. If she chooses to eat on an excursion, she'll be on her own essentially.