Fresh fruit is available for a snack credit, as is bottled water. You can get water as your beverage and can generally get fruit for the dessert (although it may be the kids meal grapes).
As for healthy entrees, it depends entirely on how she defines "healthy." Some of the salads are pre-dressed, so neither low in calories nor low in fat - Flame Tree's is, Columbia Harbor House isn't. Cosmic Ray's half-chicken with double green beans is relatively low-carb, and sticks with you a long time.
I found that if I limited my desserts and tried to avoid deep-fried stuff, I lost weight without otherwise restricting my eating. WDW is a lot of walking, and you do need to eat enough calories to carry you through.