Things to consider.
Get the building mgmt / homeowners' assoc. to be part of the solution: start by sharing this discussion with them and making it a topic to deal with. In my high rise condo it also happens that people living on the higher floors leak water onto those living on the lower floors, and (seriously, folks) I'll bet golly gee that this happens in every other building on planet Earth where gravity works. In your case it seems to be worse than in any other case I've seen or heard of. In my high rise condo building, the person living in the unit where the leak started pays for the physical work to fix damages but not any more compensation than that.
To make walls, I've used concrete-faced foam boards (e.g.Wedi) and I've used concrete boards of various kinds (some with styrofoam pellets in the concrete mix). Concrete surface can be finished with drywall compound, just like as if it were paper faced drywall. So far I am not making a recommendation ; this is just information for you to put in your pipe and mull it over.
: - )