if the cast doesn't cover the knee, the length doesn't matter.
There are some attractions that would not be possible with a long leg cast (one that covers the knee).
What will really make a difference is whether or not she can bear weight on the broken leg. It will be much more difficult if she can't bear weight.
With a wheelchair, crutches or a visible cast, she will be normally routed to the accessible boarding area of the regular one is not accessible. In that area, they can slow or stop moving walkways.
The only exceptions are peter pan and tomorrowland transit authority (peoplemover), which can't be slowed or stopped. Swiss family robinson treehouse and tom sawyer's island require guests to be ambulatory.
Some attractions will be easier to transfer to than others. Near the top of this board, there is a disabilities faqs thread, which will be helpful.
Page 2 has some posts that will be especially helpful:
Post 18: Mk attractions and accessibility. Lists which have a difficult step on and which have a wheelchair car
post 19: Epcot attractions and accessibility. Lists which have a difficult step on and which have a wheelchair car
post 20: Disney hollywood studios and accessibility. Lists which may be difficult to board and which have a wheelchair car
post 21: Animal kingdom and accessibility. Lists which may be difficult to board and which have a wheelchair car
post 22: Attractions where guests must stand unless they have mobility device
post 23: Attractions with warnings
post 28: Attractions with moving walkways and stairs
and, these on page 3
post 31: Wdw attractions with a long leg cast
post 32: Walkers, rollators, leg walkers and other mobility devices