While headphones will help I suggest you encourage your dad to return to the audiologist who fitted the hearing aids. Aids should be able clearly and intelligible hear speech, and do it under all contidions. I know, as a hearing aid wearer for over fifty years. The hearing aid technology today is more then capable of handling clear speech under every condition. Yes, there are hi-fi hearing aids and they are not expensive.
The issue really goes beyond the TV, in that he will not be hearing other sounds, like the speech of his grandchildren, kids etc., much more important then the TV.
The issue really goes beyond the TV, in that he will not be hearing other sounds, like the speech of his grandchildren, kids etc., much more important then the TV.