If this happened the way you described, I’d be furious. Leaving a cat in a room that hot while moving the AC to cool an empty room shows a shocking lack of judgment. The important thing now is that the cat is safe, but your cousin deserves to know exactly what happened so she can decide whether she can trust that roommate with her pet ever again.
I let my cousin know, that roommate moved out today because apparently it's not the first time she took things that weren't hers
If she had a history of taking stuff that wasn't hers, moving out was probably the cleanest ending possible. At least your cousin won't have to keep wondering what disappears next.