"My father has hearing loss and it's bad enough that not only does he use hearing aids, he is constantly turning them up until they feedback and start ringing."
This may also be a sign that the hearing aid wasn't fitted properly — or the shape of your father's ear has changed a bit since it was fitted. Poor fit makes it much more likely for this to happen (coz speaker and mike have a more open channel between 'em, and more ambient noise gets in so folk turn 'em up more). (My dad has similar problems until he got a new ear canal mould taken.)
It may also be a sign that you're dad's hearing loss has changed since the hearing aid was chosen and he needs it adjusting, or a different type.
Feedback is usually a sign that something needs fixing ;-)