We have found the Raid Ant Baits Plus traps are great.  The way they are
designed it is difficult at best for the ferrets to get to the poison if
they do get near the trap.  (Same thing goes for my 18 month old niece) Of
course we don't let them play with them.  Another neat thing is to pour
regular table salt around the perimeter of the room, it somehow either
repels or kills ants, and is non-toxic.  It cleans up easily with the
vaccum, and is cheap too!
 
I wouldn't suggest Borax, I believe it is either poisonous or caustic.
 
With the description of your ants, I think I would call in a pro- it sounds
like they might be either carpenter ants or termites, although it is odd to
find them on the second floor.  Still, I wouldn't take a chance.
 
You might want to have your vet check for cataracts.  Your ferret is very
young for them, so it's an outside chance, but stranger things have happened.
 
-Anne
[Posted in FML issue 2299]