You should call the ASPCA on them.  There ARE rules regarding ferret
husbantry and all it takes is one complaint to send an officer down and
have them cited and the situation corrected or the ferret removed.
 
Calling the ASPCA is easy and they are very understanding and willing to
help, I know, I've done it myself a few times.  Try to find a ferret-only
shelter that would be willing to take the furball if it is removed or
maybe take it yourself.
 
The ASPCA can't do much without a complaint, but when they get one they
can do a lot.  Please, since you're so mad, make the phone call.  You'll
be doing a little fuzzy a lot of good.  Because not only is that imroper
care for the fuzzy, whoever buys him is going to think that it is also and
likely put him in a similar aquarium.  Those types of aquariums are more
than just boring, they are bacteria incubators and can cause heat stroke
because they get so hot.
[Posted in FML issue 3655]