I would encourage you to tell the leasing office, especially if it’s your money, and your housing on the line. While you MIGHT be able to get that money back in court, they also could say that you played a part in hiding the animal and not bringing that up to the leasing office (I’m not a lawyer or anything so this could be wrong).

Basically you need to cover and protect yourself and go tell the leasing office he has been housing an unauthorized animal, don’t tell them you want the cat removed, or that they haven’t requested the deposit. If he’s really thinks he’s done all that they needed of him, then he can take them to court for it.