HIV cannot live outside of a host for any more then a very short period of
time.  Animals aren't carriers of HIV--otherwise, I'm sure we'd hear from
hysterical people over dog bites.  Remember back when people thought you
could become infected through mosquitoes or flea bites?  That was proven to
be completely untrue.  Your ferret will be fine even if he bites an infected
person, and he will not be able to pass the virus on.
 
As far as your mom's health goes, she probably wants to play it safe and not
clean the litter boxes (due to the very small risk of toxoplasmosis) and be
careful around sneezy ferrets, as some colds can go back and forth.  I've
known a few HIV+ people to have ferrets and have had no problems with them.
 
Feel free to contact me directly if you want some more scientific
information or citations.
 
Lela
Health Educator
[Posted in FML issue 1797]