FYI for all - there is an international organization called Best Buddies that matches typical people with people with disabilities to include them. It started in high schools, but it is expanding to the adult level. Anyone who wants to volunteer with them should go to bestbuddies.org.

I have a kid with DS and I can attest that there ain't no party like a Down syndrome party.

One sheet of drywall probably won't cost much more than a big bucket of plaster/joint compound. The plaster is obviously pulling away from the lathe. I would just put up a sheet now.