First, all of the cruise lines work well with food allergies and other dietary requirements (vegetarian, vegan, etc.). On the cruise line websites there is always a link to the special needs section where you should alert them to her needs. Once on the ship, speak with the maitre ‘d in your dining room. In the buffet, ask to speak with the cook.
As far as choosing a cruise, I like to use cruisetimetables.com. You can research cruises by departure port (cruises from…), or a port you want to visit (cruise to…). It will show you the ships and their itineraries.
It’s a great site to wander around in. EM