Check out my allergy page for answers to most of your questions....in
summary ... yes you can have A ferret (or two) without hurting your father
IF you follow strict precautions listed on my site.
 
Not listed are things like using an aircleaner.  Not listed on there is
also how one thing can trigger sensitivity to other allergens that don't
normally bother you.  So in short get a handle on everything else first.
Get a handle on dusting constantly, vacuuming with a dust free vacuum every
day, possibly using dust mite killer regularly on his bedroom carpet (where
ferrets won't go), keeping air conditioning on (and the filters cleaned),
going to walmart and buying for 12 bucks the plastic zippered mattress
cover to protect from dustmites (you can throw a mattress pad, or thick
cover over it and under his sheet so it won't bother him), limit your
plants in your home, there are zillions of precautions to take that would
help.  Above all...and please no one take offense...do NOT smoke in the
home.
 
I hope my site helps you some.  And I think a ferret would be a lovely
thing that can actually help the mental health, etc of anyone... :).
 
Wolfy
Please visit:
http://www.geocities.com/wolfysluv/
for information on deafness and ferrets:
http://www.geocities.com/wolfysluv/deaf.html
take a peek at upcomming "so you have allergies":
http://www.geocities.com/wolfysluv/allergies.html
[Posted in FML issue 3162]