I do not live in Chicago, and I wouldn't want a rat if I did. But to convince your mom, you could pull the pity card. Tell her that it's not right that the guy is mistreating his rat and that you wanna take it in until you can find it a permanent home. It can stay in your room, won't get loose, and you'll take care of it. Your mom won't even have to see it, unless she goes in your room.