Some people might not know this, but Arianna actually just passes out when you kill the infant Great One. The animation she performs is not a death animation, you can see this if you were to attack her before the nightmare infant or both at the same time.
I did notice that when you warp back to the chapel and go to see her again, her position does change to a dead one, but I think ot's because of how to game loads enemies and npcs when reseting the area. Her initial animation is evidence enough that she's not dead, just unconcious.
Guess it makes sense that you can take her shoes (weird choice) but whatever.