I question whether LW can conceal the fact that he now has a cat. Even if he can put away the litter boxes and scratching posts where his parents won't find them, a missed toy or some stray cat hair will give him away. He could tell his parents he has a cat without revealing that it's their cat, but then he'll have to explain—for years—why the cat is never at home when they visit.

LW's parents abused that poor cat, and in my opinion, LW has no moral responsibility to say anything. He has my blessing to lie if he can pull it off. But I doubt he can.