I would say that the roommate should bear the total cost of the repairs, full stop.
Roommate was warned before moving in that they specifically shouldn't leave glasses of liquid unattended, because of the cat. They agreed to this in advance of moving in. They then left a glass of liquid, unattended for a long period of time, right next to their pricy, submerging in liquid voids the warranty electronics. The fact that liquid got spilled on it was a logical and not at all surprising result.
In general, I would say that the person who was responsible for the liquid/electronic combination should be responsible for repairs. So if someone had their laptop sitting out, and someone else put a full glass of water next to it and left it unattended, the person who put the water out would be responsible. But if someone left a glass of water out, and someone else came up and put their iPhone next to the water and wandered off, the person who left the iPhone would be responsible. On the other hand, if you leave your iPad on the kitchen counter and water gets splashed on it then it's your own fault, for leaving the iPad in the middle of a splashy area of the house.