She should have plenty of time to learn about changes she needs to make as far as food intake (less overall, especially anything carb/sugar -- I didn't know that peas and corn are considered carbs in a diabetes diet until I went to the class), and all the extra walking exercise at WDW will be helpful. She will need to keep an eye on what and when she eats (smaller meals/snacks a little more often) and keep her fluids up (water, water, water).
I'm sure you'll get lots of helpful advice. With a few simple adjustments, she should be just fine and able to have lots of fun. (My sister's doc gives her extra slack when she's on vacation -- an occasional treat or drink can be okay). You could do a little reading about diabetes so you get familiar with what she needs to be aware of (but don't be her "police-person" -- just remember that a little food a little more often (healthy snacks -- you can take food into the parks) is advisable to avoid "wooziness" -- don't skip regular meals or overdo it).
Since WDW is such a high-carb/sugar environment, it can seem a little daunting -- but easily doable really (maybe there's even a game opportunity in being able to resist all the temptations). (And in the interest of being supportive, you may want to avoid a few treats, as well (but not everything -- she won't want to spoil your fun)).
