Sounds like the typical hair loss pattern for adrenal gland disease.  PLEASE
download the "Adrenal Gland FAQ" from the same listserver that drives the
FML; BIG, can you aim it at her?  Show it to your vet; it doesn't sound like
he's all that ferret knowledgable but I can't be certain of that.
 
It's my understanding that "bad adrenal glands" are sometimes cancer
related, other times not - but even when cancerous, the tumors seldom spread
away from the adrenals, so it's usually fixed with surgery.
 
Note: I'm no vet, Felix is dealing with the same problem.
 
[Moderator's note The file to ask for is ADRENAL DISEASE.  If you have
trouble getting it please let me know and I'll send it.  BIG]
[Posted in FML issue 1612]