I've read that cat litter is bad for a rat, but what if they're not actually living in it? My friend has a rat who lives in a large wire cage and there is a removable tray she fills with cat litter underneath it so he never even touches it. (only half the floor is open to the wires, the rest is covered, as is the second layer). Do you think that's ok? He doesn't seem to be suffering for it and she's had him for almost a year.