Troy boss fight is bugged

One of my friends can’t kill troy, anytime she goes into the arena in their world troy is floating in the air over tyreen and cannot be damaged and he never leaves the spot above her. We’ve tried quitting out of the game and restarting it, fighting him in my world, leaving the area and coming back but when she goes back to try and fight him for her story mission the cutscene never plays and he just floats there unable to be damaged.