How old is your cousin?? I am NOT the sort of person that would normally advise to go against parental authority...but when that authority is being abused, sometimes it can't be helped. If your cousin is just a little kid, then he's really powerless in this situation. But if he's an older teen, he is plenty old enough to take responsibility for his own pet. IF the kid is old enough to be aware of the consequences of his choices and the responsibilities he bears himself, have HIM give you the snake behind his mother's back.
He can tell his mother the snake died (or escaped) or whatever...maybe she wouldn't even notice it was gone if no one mentioned it?Don't worry about trying to sneak out some fish tank....if you want to rescue the snake, buy your own setup for him and have the cousin sneak just the animal out of the house in a small, secure bag.