The answer is going to depend a lot on her personal preferences. Water stops are usually about every two miles or so, which is frequent enough for me that I never carry water at RunDisney events. April can be hot in Florida though, and lots of people do tend to bring their own water or other hydration drinks with them, either to drink more frequently or just to avoid the crowds around the water stops. If she decides to carry fluids with her, there are lots of options - waist belts with bottles, hydration backpacks (Camelbak style), or handheld runner's bottles with hand straps. She can visit her local running store and see what works best for her. I would just recommend she try using whatever she selects on a few training runs first, just to get used to it and make sure it doesn't cause any rubbing issues or anything. I've used belts, backpacks, and handhelds on various occasions, and it's a little different at first, but most people get used to them easily.
Best of luck to her on her first half marathon! I've done Dark Side before and it's a great race weekend. Plus, Disney runners are a great bunch - really fun and friendly people who make the races awesome experiences. Tell her to be on the lookout for all the great costumes. That's one of my favorite parts of Disney races!