Are they considering a trip at Christmas this year (next month)? If they don't already know, they should be made aware that the time between Christmas and New Years is probably the most crowded time ever at WDW. Crowds will be elbow-to-elbow everywhere. If they don't already have a reservation, they may have trouble finding one; and dining reservations for table service meals are probably just as hard to get at this point. Earlier in December is less crowded and still nicely decorated for the holidays. Some people simply love going at that time of year, but if they aren't aware of the crowds it could be a bust for them.
If the girl needs certain seating at shows that can be requested at each attraction. I don't know if she regularly uses a white cane, but it has been suggested to carry one anyway as a visual cue to others. Did you have other accommodations in mind? Does she have any vision? Would an audio description help? If she has some vision, does she have trouble going from light-to-dark (such as entering a building or ride queue)?
Personally, I would not want to take a blind child to WDW right at Christmas due to the crowds. But some people love the atmosphere and tolerate the crowds by taking it slowly without worrying about missing stuff -- they may also take a long enough trip or go back often enough that the rides aren't a major priority at Christmas. If this family wishes to go at Christmas, I'd suggest next year rather than this year so they have time to plan.
