Hi Julie!
Thank you for bringing your question to the Disney Parks Moms Panel!
I am sorry to hear that someone has an injury requiring a cast. As the mother of two teenage boys, I have seen (and smelled) my fair share of casts, so I know that they can be cumbersome and hard to deal with.
Your cast should be fine for almost all the attractions at
Walt Disney World Resort as long as you're able to fasten the safety harness for each. However, you may want to avoid the rides where you'll get wet (
Splash Mountain,
Kali River Rapids, etc.) Once when my son was in a cast, we used a plastic grocery bag over his for those rides so he wouldn't miss out. We just packed some bags and thick tape in our backpack, then wrapped him up before getting in line. It worked perfectly!
I hope this helps! Please let me know if you have any other questions and I wish a speedy recovery for the injured person.